Tag helpers attribute code generation

- Followed the "decorate" pattern that we use throughout Razor.
- Enabled the CSharpCodeBuilder to decorate the CSharpCodeVisitor which exposes a CSharpTagHelperCodeRenderer which is indirectly used to render attribute values.
- Fixed up some existing classes that just new'd up the CSharpCodeVisitor class to instead take in the decorated version.
- Tested that we can successfully plugin our own TagHelperAttributeCodeGenerator.
- Moved some existing TagHelper test logic into its own base type to enhance code reusability.

#119
separately `DecorateCodeBuilder()` should really be `CreateCodebuilder()` as you're using it here.  tip-off is ignoring `incomingCodeBuilder` and not needing to call `base.DecorateCodeBuilder()`.
fine if this can't be changed due to other `DecoratecodeBuilder()` overrides.  but may still be worth tracking as a separate issue if not addressed in this PR.