Feature: Generated button for sample option pane

[Arlodotexe]

Summary

This PR contains all necessary changes to close #4, and some additional quality of life changes.

Refactored SampleGen source generator

• Enables getting attributes attached to method symbols
• Adds the ToolkitSampleButtonActionAttribute and all the necessary plumbing to use it (see below)

Refactored SampleGen test tooling

Until recently, tests only checked the diagnostic output of a source generator, not the actual generated code. This changed with CommunityToolkit/Labs-Windows#336 when we added the PaneOption_GeneratesTitleProperty test.

• This test also needed to check the sample registry, which is generated in the app head instead of the sample project.
• This resulted in a copy-pasted method that worked well for that test, but not for the new tests.
• We needed to properly recreate the Head-Sample project reference setup we have in our solution. These changes now enable that.

WIP: Pending issues

This PR is still work in progress. There may still be some functional bugs and missing plumbing:

• Source generator sometimes doesn't see the button attribute. Cause unknown, seems random.
• When the button does generate:
  • Binding to the command in your own sample should work as expected
  • Clicking the button in the generated options pane will do nothing. This is because:
    • The sample registry is a single tree of objects created at the app head
    • The same sample data is provided to all controls
    • The data does not have access to the control consuming it, and thus has no way to call the instance method for our button.

[Arlodotexe]

@michael-hawker Looks like C# 11 isn't supported by the .NET 6 SDK? We need this for string literals """.

[michael-hawker]

@michael-hawker Looks like C# 11 isn't supported by the .NET 6 SDK? We need this for string literals """.

Yeah .NET 6 = C#10, .NET 7 = C#11, so we'd need to bump the version in the build.yml/global.config to be installed/used for build. Should be fine, we can still target .NET 6 so, none of the other stuff should have to change.

[Arlodotexe]

This PR is experiencing a major blocker related to the source generator. The generated button option has been removed and the remaining changes and improvements have been moved to #69.

As a substitute, we'll use a custom options pane that can hold any sample options we need.