Add IsTrimmable annotation

[brantburnett]

Motivation

When trim-compatible constructs are used we should mark the generated assembly as trim-compatible.

Modifications

Automatically mark generated assemblies as trim compatible when:

• The target is .NET 6 or higher
• All extensions used are trim compatible, or no extensions are used

For now, only the MicrosoftExtensionsHttp extension produces trim-compatible code.

Results

Assemblies produced without JSON serialization are trim-compatible. Further work will need to be done to make System.Text.Json compatible.

[brantburnett]

/merge