Default Interface Implementation "ShouldOverrideAttribute"

[NinoFloris]

I propose roslyn should respect an attribute like "ShouldOverrideAttribute" that signifies a specific default interface method should rather be re-implemented instead of left as a default.

I foresee this being almost necessary when you version an existing interface to include a new method where you don't have a sane default implementation besides throwing.

When the attribute is applied csc (or maybe just analyzers) can warn or hint implementation authors when they re-compile against the newer interface that there is a method they should probably implement.

[gafter]

If this is to be required by the language and enforced by the compiler, then this is a language feature request and belongs in the csharplang repository.

If it is instead intended to be enforced by tooling (i.e. the IDE), then this should be reassigned the Analyzers or IDE area.

I assume the latter was the intent, so assigning to Analyzers.