refactor(client): de-duplicate process & run and split into h3 & h09

The Neqo Client binary supports both http3 and http09 (prev. "old").

Before this commit both the http3 and the http09 implementation had their own
`run` and `process` `fn`, orchestrating the interaction between handler, client
and I/O. While similar, they had subtle differences e.g. when to terminate.

This commit splits the http3 and http09 specific logic into two separate modules,
but extracts duplicate logic (e.g. `run` and `process`) into the shared root module.

neqo-bin/Cargo.toml

@@ -11,7 +11,7 @@ license.workspace = true
 
 [[bin]]
 name = "neqo-client"
-path = "src/bin/client.rs"
+path = "src/bin/client/main.rs"
 
 [[bin]]
 name = "neqo-server"