Support github issue forms for "Ticket" feature

[petersutter]

What this PR does / why we need it:
Support github issue forms for the ticket feature. For more information on github issue forms, refer to the GitHub documentation

The table below outlines the properties nested under Values.global.dashboard.frontendConfig.ticket.newIssue in the gardener-dashboard Helm chart configuration:

Property	Type	Description	Example
title	string	The title of the issue. If not defined, it will default to [${shootNamespace}/${shootName}].	title: "[${shootNamespace}/${shootName}] <problem>"
labels	array	Labels to be added to the issue.	labels: ["bug", "urgent"]
template	string	The name of the issue template to use.	template: "bug_report.md"
body	string	The body of the issue. This should not be set if you are using GitHub issue forms.	body: "Steps to reproduce the issue..."
<form-element-id>	string	The ID of the form element. This can be pre-filled with the appropriate value.	affectedCluster: "${shootUrl}"

Note: Any field under newIssue will be used as a query parameter when creating a new issue.

Available template variables:

• ${shootName}: name of the shoot
• ${shootNamespace}: namespace of the shoot
• ${shootCreatedAt}: creation timestamp of the shoot, format 'YYYY-MM-DD'
• ${shootUrl}: dashboard url of the shoot
• ${providerType}: shoot provider type
• ${region}: region of the shoot
• ${machineImageNames}: comma separated list of (unique) machine image names from the shoot workers
• ${projectName}: name of the project
• ${utcDateTimeNow}: current date-time in utc format
• ${seedName}: shoot's seed name
• ${accessRestrictions}: list with shoot's selected access restrictions
  • accessRestrictions[].title: access restriction title
  • accessRestrictions[].description: access restriction description
  • accessRestrictions[].key: unique identifier
  • accessRestrictions[].options: list with selected options for the access restriction
    • accessRestrictions[].options[].title: option title
    • accessRestrictions[].options[].description: option description
    • accessRestrictions[].options[].key: unique identifier

Each value of a property under newIssue is treated as a template and the above-mentioned template variables will be applied during runtime.

Sample GitHub issue form:

name: "Sample Issue Form"
description: "Guided creation of an issue for a Shoot cluster"
title: "[<cluster>]: <problem> "
labels: ["topology/shoot"]
body:
  - type: markdown
    attributes:
      value: "# Gardener help center "
  - type: markdown
    attributes:
      value: |
        Please provide metadata and information for this issue, such as specifying which cluster is affected.
        By categorizing and prioritizing the issue appropriately, you can help us process this ticket more quickly.
  - type: input
    id: affectedCluster
    attributes:
      label: "Which cluster is affected?"
      description:  Please provide the link to your cluster in the Gardener Dashboard, so that we can identify it and run some self-diagnoses upfront.
    validations:
      required: true
  - type: textarea
    id: howToReproduce
    attributes:
      label: "How can we reproduce the issue (concisely and precisely)?"
      placeholder: |
        1. 
        2.
        3.
    validations:
      required: true

Example values of the gardener-dashboard Helm chart:

Create Ticket using GitHub issue forms

global:
  dashboard:
    frontendConfig:
      ticket:
        gitHubRepoUrl: https://foo-github.aaakk.us.kg/dummyorg/dummyrepo
        newIssue:
          fileName: shoot_issue.yaml
          affectedCluster: "${shootUrl}" # example template form element id

Create Ticket using regular GitHub issue:

global:
  dashboard:
    frontendConfig:
      ticket:
        gitHubRepoUrl: https://foo-github.aaakk.us.kg/dummyorg/dummyrepo
        newIssue:
          body: | # do not set `body`, if you use GitHub issue forms
            ## Which cluster is affected?

            `Cluster Details Dashboard Link`: [${projectName}/${shootName}](${shootUrl})
            ...

Which issue(s) this PR fixes:
Fixes #1638

Special notes for your reviewer:

Release note:

Changes have been made to two `gardener-dashboard` Helm chart values related to the ticket feature:
- The value previously at `Values.global.dashboard.frontendConfig.ticket.newTicketLabels` has been moved to `Values.global.dashboard.frontendConfig.ticket.newIssue.labels`.
- The value previously at `Values.global.dashboard.frontendConfig.ticket.issueDescriptionTemplate` has been moved to `Values.global.dashboard.frontendConfig.ticket.newIssue.body`.

GitHub issue forms are now supported when creating new tickets (GitHub issues) from the dashboard.
- Set `Values.global.dashboard.frontendConfig.ticket.newIssue.template` to specify the filename of the issue form template. For more information, refer to the [GitHub documentation](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository#creating-issue-forms)
- To pre-fill template form elements, specify the `id` of the form element under `Values.global.dashboard.frontendConfig.ticket.newIssue`. For instance, if you have a form element with `id` `affectedCluster`, you can pre-fill it like this: `Values.global.dashboard.frontendConfig.ticket.newIssue.affectedCluster="${shootUrl}"`. The template variable `shootUrl` will be replaced during runtime with the actual dashboard link of the shoot cluster.

[gardener-robot]

@holgerkoser, @grolu You have pull request review open invite, please check

[grolu]

/lgtm

[holgerkoser]

/lgtm