Ability to register your own content provider for default template content

[matthewcare]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Added functionality which allows you to register your own "default view content provider" for custom content generation when creating new templates in the back office.

Updated all tests, and marked the old method obsolete. I didn't remove it, as the class is public, and someone could have been using it.

It's my first time going this deep into the Umbraco source code, so if there are any files out of place, or not seeming to be in keep with the style, let me know and I'll move things about.
I put the registrations in a builder file that seems to be in keep with "file" stuff, I hope it's correct,

Testing

Create your custom provider

public class CustomViewContentProvider : IDefaultViewContentProvider
{
   public string GetDefaultFileContent(string layoutPageAlias = null, string modelClassName = null,
                       string modelNamespace = null,  string modelNamespaceAlias = null) => "super content";
}

and then register it

services.AddUnique<IDefaultViewContentProvider, CustomViewContentProvider>();
or as part of an IUmbracoBuilder
builder.SetDefaultViewContentProvider<CustomViewContentProvider>()

Create some templates in the back end, and verify the output is correct.

[umbrabot]

Hi there @matthewcare, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
• The contribution looks original and the contributor is presumably allowed to share it

Don't worry if you got something wrong. We like to think of a pull request as the start of a conversation, we're happy to provide guidance on improving your contribution.

If you realize that you might want to make some changes then you can do that by adding new commits to the branch you created for this work and pushing new commits. They should then automatically show up as updates to this pull request.

Thanks, from your friendly Umbraco GitHub bot 🤖 🙂

[bergmania]

Looks good, but there are a breaking change we need to handle..

[bergmania]

Thanks @matthewcare 💪