Add a registry of supported signature suites/hash algorithms

[woodruffw]

Opening as a tracking issue; CCing @tetsuo-cpp and @ret2libc.

From initial discussion, protobuf-specs is probably the right place for this registry to live. It should probably be both a Markdown file and a .proto (containing enums/string constants for the cipher suite names, maybe?)

Blocked on completion of the relevant parts of the Configurable Crypto Algorithms document.

[woodruffw]

Some relevant discussion items:

• How "detailed" should the protobuf version of the registry be? In other words: should it list things like expected purposes (signing only, verifying only, both), or just be a "bare" list of every algorithm and suite that a Sigstore client might reasonably need to support?

[tetsuo-cpp]

This is addressed in #194, we can close this now.