[API Compat] Add rule to make sure nullable annotations are compatible

[safern]

This rule should make sure that nullable annotations match in between the implementation and the contract.

Also, when not running on strict mode, this rule should allow differences in nullable annotations that are not breaking, for example, relaxing a nullable reference type annotation on an in parameter (i.e public string Foo(string a) -> public string Foo(string? a)).

cc: @ericstj @Anipik

[dotnet-issue-labeler]

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