Add TLS support for diesel-async database connections

[sunaurus]

This PR adds a new tls_enabled configuration option in the database block of lemmy.hsjon. When configured to true, Lemmy will create SSL connections to the postgres database through diesel-async.

By default, the database certificate will be verified using the OS certificate store (thanks to rustls-native-certs), but users can also provide a path to a CA certificate file directly using the LEMMY_DATABASE_CERT_PATH env variable if necessary.

Note, by default tls_enabled will be false, so using SSL connections will be entirely opt-in.

This fixes #3007

[sunaurus]

I have significantly reduced the scope of this PR: it only aims to provide support for sslmode=require now (which means that a TLS connection will be used - preventing eavesdropping - but the server identity will not be verified).

No new configuration parameters are needed anymore, sslmode=require is parsed from the database connection string.

After discussions with some other admins, I realized that sslmode=require support would provide the biggest ROI while keeping the code much simpler compared to trying to support additional sslmodes.

[tristanisham]

I see what you did here 👀