Make `openSSL` opt-out when using `pq-src`

[stormshield-gt]

Would you be open to not include systematically openssl when using pq-src ?

My use case is to use rustls-ssl with libpq, without needing to have openssl as a dependency. @ctz has made a successful POC showing that this is possible recently.

I don't know when this POC will be ready to use, maybe when rustls-ssl will be more stable, and I don't want to add any pressure to do so. I just want to see If you will be open to this, so we could maybe start preparing for it.

Another use case is if the user wants to use the bundled libpq but still provide their own openssl bin.

[weiznich]

I'm happy to accept a PR that:

• Moves openssl bundling behind a feature flag
• That uses rustls-ssl behind a different feature flag

I would such a PR to extend the CI test matrix + contain some documentation on the relevant features.

[weiznich]

I've released pq-sys 0.6.0 which does not link OpenSSL statically by default.