Versioning for Client/Server

[fkettelhoit]

Now that the repository is public, we should start thinking about how we want to handle a version mismatch between the client and server. Since the playground (or rather the echo server) is auto-deployed whenever something changes on main, everyone using a previously installed tandem_http_client will simply get a very confusing error message whenever a change results in incompatibility:

tandem_http_client --function main --input 3u8 --metadata 7u8 add.garble.rs
Error: MaxRetriesExceeded([ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}"), ServerError("{\"error\":\"BincodeError\"}")])

I would propose 2 changes:

• only deploy new versions to fly.io when a new tag is pushed (not on every commit)
• check the client version against the server version, print a more helpful error message in case there's a mismatch