Common traits on builders for polymorphism

[JakeWharton]

It's really hard to extract code which can operate on two different spec builders.

Off the top of my head, I strongly need:

• The ability to add annotations (types, properties, functions)
  • Extract trait interface for annotatable constructs and their builders #1564
• The ability to add kdoc (types, properties, functions)
  • Extract trait interface for documentable constructs and their builders #1571

Other probable inclusions:

• The ability to add types (files, type)
  • Extract interface for constructs that can hold a TypeSpec and their builders #1723
• Maybe the ability to add functions and properties (file, type)
  • Add MemberSpecHolder #1867
• The ability to add code (CodeBlock, fun spec)

I don't think we need a full class hierarchy similar to PSI/FIR, but trait-like interfaces for these fairly primitive operations would be welcome. Maybe named like AnnotatedSpecBuilder, DocumentedSpecBuilder, etc.

[ZacSweers]

We've done a bit of stuff around Taggable and Context Receivers. Could that be a good starting point?

[JakeWharton]

I've never used either and don't recall their implementations. So, maybe?