Multi-version client 2.0

[sfc-gh-anoyes]

This is a proposal to make obtaining a compatible libfdb_c easier for clients.

• [x] Add the concept of stable endpoints to flow transport
• [ ] Add a symbol to the c api to expose the version of the cluster
• [ ] Only run the compatible client for each database in the multiversion client
• [ ] Add support for downloading libfdb_c from the server over a stable endpoint. (TODO: How should this look in the c api? Should this happen transparently? Should there be a new symbol that downloads libfdb_c to a particular path)
• [ ] Support adding a client to the multiversion client while it's already running (if that's not already supported)
• [ ] Add the ability to upgrade a connection's checksum to the most recent protocol supported by both peers

@sfc-gh-rchen

[alecgrieser]

Does #5694 complete the fourth bullet point (i.e., "add support for downloading libfdb_c")? Or is the endpoint added there not stable across versions (presumably left for future work)?