This adds [removable feature annotation](dotnet/designs#42) to `XmlDownloadManager`.

If the user specifies that they would like to remove support for this at publish/native compilation time, the linker/compiler is going to replace the method body of `CreateWebRequestOrThrowIfRemoved` with a throwing method body. The exception message is going to inform the user that the feature has been removed because they opted into the removal.

Contributes to #30597. Saves 1.2 MB of the size of the Windows UWP People app. This is a size on disk regression that came with NetStandard 2.0.

…t#31533)

This adds [removable feature annotation](dotnet/designs#42) to data
contract serialization.

If the user specifies that they would like to remove support for this
at publish/native compilation time, the linker/compiler is going to
replace the annotated method bodies with a throwing method body.
The exception message is going to inform the user that the feature
has been removed because they opted into the removal. The throwing
method body is significantly smaller than the transitive closure
of code reachable from the original method body.

Contributes to #30597. Turning this feature off in the UWP People app
saves 4% of size. This is a size on disk regression that came with
the new version of CoreFX and blocks the Windows team in picking up
the latest framework. There is a zero size growth goal.

This ports over the change from
dotnet/corert@0ac83cb.

When UsingResourceKeys is true and we stripped the resources, the
existing code would have returned null.