Add support for TLS when connecting to servers

[pinaraf]

Is your feature request related to a problem? Please describe.
PgCat doesn't allow connecting to a PostgreSQL server that requires TLS.

Describe the solution you'd like
PgCat should be able to send an SSLRequest message before StartupMessage to request TLS from the PostgreSQL server, thus making it usable even on "untrustable" networks.

Describe alternatives you've considered
None

Additional context
None

[levkk]

There is an example on how to do that with the Client in client.rs. We'll just need to make Server generic the same way and negotiate a TLS connection 👍