Add HasSerializationOverrides

[jackkoenig]

This is to help code relying on annotations limp along, not really intended for normal use.

This is technically a new API but I don't want it to be treated like a feature in the release notes.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• API deprecation

Desired Merge Strategy

• Squash

Release Notes

This is a deprecated API in firrtl.annotations that is similar to HasSerializationHints but allows for overriding the serialization class like OverrideSerializationClass does for Annotations themselves.

Also remove the Annotation self typing on OverrideSerializationClass because it is pointless.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash) and clean up the commit message.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.