arti-rpc: Implement rpc-connect-sketch.md

This was once arti-rpc-client-core: Make "connect string" a json object., but since we started this design, the requirements have grown a lot.

• Make a specification
• Initial connect-point backend implementation. (!2629 (merged))
• Adjust fs-mistrust to support more patterns of opening files. (#1746 (closed))
• Implement client-side logic in arti-rpc-client-core.
• Implement server-side logic in arti-rpcserver.
• Implement cookie authentication (#1521 (closed))
• Integration tests (general)
• Integration tests for RPC decline/abort behavior, and fixups for any gaps.
• Resolve all TODO RPC entries.