Move the metadata update related APIs to the MetadataUpdater class

[mikem8361]

The old APIs AssemblyExtensions.ApplyUpdate() and AssemblyExtensions.GetApplyUpdateCapabilities() will be removed
after all the references to them have been changed to the new APIs.

Add the new IsSupported API described in issue #51159.

Change the tests to use the MetadataUpdater APIs.

/cc: @pranavkm, @eerhardt

[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.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[mikem8361]

@lambdageek, I'm sure the mono version of MetadataUpdater.IsSupported isn't complete, but there is a place holder.

[lambdageek]

Mono bits LGTM. CoreCLR looks ok, too, but I'm not super-familiar with the code.

One thing to watch for: the ApplyUpdateTest testsuite only runs in environments where ApplyUpdateUtil.IsSupported is true - for CoreCLR that's basically everywhere, but for Mono in environments where we can't use the remote executor (ie mobile and webassembly) the whole class will only run when DOTNET_MODIFIABLE_ASSEMBLIES is already set appropriately by the launcher.

So MetadataUpdater.IsSupported will always return true if the AssemblyUpdateTest class is not skipped under Mono.

[mikem8361]

All I'm waiting for now is for @eerhardt to approve the feature switch changes and for the API review for making MetadataUpdater.GetCapabilities() public.

[eerhardt]

LGTM

[stephentoub]

We should also update

runtime/src/libraries/System.Private.CoreLib/src/ILLink/ILLink.LinkAttributes.Shared.xml

Line 34 in b1a4178

<type fullname="System.Reflection.Metadata.MetadataUpdateHandlerAttribute">

to be based on the new switch rather than on Debugger.IsSupported.

[mikem8361]

How would one make the attribute do that? The xml file just seems to be the attribute type.

[stephentoub]

It's currently in the section for the debugger switch:

runtime/src/libraries/System.Private.CoreLib/src/ILLink/ILLink.LinkAttributes.Shared.xml

Line 3 in b1a4178

<assembly fullname="System.Private.CoreLib" feature="System.Diagnostics.Debugger.IsSupported" featurevalue="false">

A new section should be added for the new switch and this attribute moved to that section.