Developer-facing documentation for our protocols

[jleibs]

I'm struggling to understan what our actual wire protocol looks like for:

• RRD streamed over http
• Messages streamed over tcp

I don't currently understand what aspects of the framing are shared and/or how they diverged.

Example of the kind of questions I'd like to be able to answer:

• Do we even need a tcp/client PROTOCOL_VERSION, or is it immediately followed by the RRD_HEADER / FileHeader by virtue of buffered_client using re_log_encoding::encoder::encode_to_bytes under the hood?
• Does Surface SDK version info from channel into StoreInfo #6280 make sense, or do we need a third RRD-schema-version encoded in the StoreInfo instead?