Allow template in pull request alert comment

[eleftherias]

Summary

Change the pull request alert comment parameter from a string to a template.
This allows us to set the comment to "{{ .EvalErrorDetails }}" which gets details from the evaluation error.

Ref #5120

Question

I'm undecided about how to structure the template params, whether it should be a nested eval object (.Eval.Error.Details) or a single string (.EvalErrorDetails). Let me know if you have a preference.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

In a rule set the alert message to a template

  alert:
    type: pull_request_comment
    pull_request_comment:
      review_message: "{{ .EvalErrorDetails }}"

This is an example from the legacy Trusty rule using the pr comment alert:

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 55.356% (+0.01%) from 55.345%
when pulling 3df29a0 on eleftherias:pr-alert-comment-template
into b69d87d on mindersec:main.