Add MetadataUpdateHandlerAttribute

[stephentoub]

And at least the beginning of reflection cache clearing support.

Fixes #49361

cc: @tommcdon, @mikem8361, @steveharter, @pranavkm

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[mikem8361]

Other than a one comment, it looks good.

Looks like you also did the reflection caching clearing too. Thank!

[stephentoub]

@pranavkm, I was just looking at dotnet-watch to see what we'll need to do there to respect this, and it seems like we likely first need Microsoft.CodeAnalysis.ExternalAccess.Watch.Api.WatchHotReloadService.Update augmented to include type names, then we can modify the manager to forward those to the agent, and then the agent can use those to resolve the Types to be passed to the update handlers? Or is the plan to parse the metadata delta in the agent to extract updated type information?

[eerhardt]

LGTM - just one comment about updating the ref file.

[jkotas]

It should be enough to just set the GCHandle target to null. The GCHandle is weakhandle and so the code should be generally prepared to handle the null case.

#51070