Clean up gRPC spec.md

[BlaineEXE]

A lot of the spec.md file is out of date from what is in the KEP.

Where possible, the KEP should be the source of truth, so I think a lot of information can just be deleted. For example, terminology, diagrams, objective, goals, non-goals, architecture. All these things should be defined by the KEP.

There are also some old things present which are no longer in the COSI KEP like AnonymousBucketAccessMode. These items probably should be removed from the spec, but they can be captured as a feature request to plan/design in the KEP in the future.

There are also areas where the spec just seems out of date with what is in the KEP. While I generally expect that the spec in code may have some minor details and bug fixes defined that aren't in the KEP, we should strive to keep things consistent.

A good time to clean up the spec.md is probably when we are updating it to be in line with the in-progress v1alpha2 design doc.

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