Support for setting client certificate and key from bytes

[wyhaya]

This PR adds ssl_client_cert_from_bytes and ssl_client_key_from_bytes to MySQL and PostgreSQL.

This will allow slqx to configure tls cert without going through the filesystem. This can be useful in certain scenarios, such as reading certificates from Keychain.

[abonander]

These methods should use _from_pem() instead of _from_bytes() to match:

• https://docs.rs/sqlx-mysql/latest/sqlx_mysql/struct.MySqlConnectOptions.html#method.ssl_ca_from_pem
• https://docs.rs/sqlx-postgres/0.7.1/sqlx_postgres/struct.PgConnectOptions.html#method.ssl_root_cert_from_pem

[abonander]

The documentation and examples could be clearer.

[abonander]

Ugh, every time I try to commit suggestions to a PR I end up breaking it more, lol.

I'll fix it.