[ExtensionsMetadataGenerator] Improve configuration for cleaning build output

[brettsam]

Today we have an "all or nothing" approach to cleaning your build output and disabling this involves a (currently) undocumented MSBuild setting: _FunctionsSkipCleanOutput. We're seeing a handful of customers run into this -- they move forward to a newer version of an assembly that we're cleaning away.

This often fails with Could not load file or assembly... exceptions during runtime.

I think we need to:

1. Make this setting not as "internal" (remove the underscore) and document this... I'd assume here: https://docs.microsoft.com/en-us/azure/azure-functions/functions-develop-vs
2. How fancy we can get with the cleanup and still have it be maintainable? Can we only delete if the version is to an exact match to what we have in https://github.com/Azure/azure-functions-host/blob/dev/tools/ExtensionsMetadataGenerator/src/ExtensionsMetadataGenerator/runtimeassemblies.txt?
3. If we can't do this smart enough with number 2, allow you use MSBuild items to selectively exclude or include assemblies. That way you can still benefit from the cleaning but get things running.

Some related issues that have come up (I'm resolving all of those to this one):

• Could not load file or assembly System.IdentityModel.Tokens.Jwt, Version=5.6.0.0 azure-functions-vs-build-sdk#397
• version 3.0.4 removes required dependencies azure-functions-vs-build-sdk#412
• Unable to start V3 Azure Function : Could not load file or assembly Azure-Functions#1518 (comment)
• Unable to load assembly after upgrading Microsoft.NET.Sdk.Functions to v1.0.33+ Azure-Functions#1525

[brettsam]

@fabiocav -- I'll sync with you on what we can do here.

[ramondeklein]

I'll share my workaround from #1525 for people who run into this issue:

It seems that the RemoveRuntimeDependencies task removes this assemblies (source). It doesn't run when the _FunctionsSkipCleanOutput variable is set to true. Adding the following line effectively disables this task and effectively disables the cleaning completely.

  <PropertyGroup>
    <_FunctionsSkipCleanOutput>true</_FunctionsSkipCleanOutput>
  </PropertyGroup>

The only downside is larger deployments...