Support SSL server certificate parameter for MySQL (#15172)

Adding ssl-cert config field to MySQL DB details map, to hold the server cert chain in PEM format (similar to what is done in MongoDB driver)

Updating MySQL driver init to map :ssl-cert into :serverSslCert for the JDBC url, when ssl is in use and cert is provided (the MariaDB driver we are using accepts PEM format certificates inline directly for the param value, so no need to shepherd into a temp file)

Adding new test to mysql_test.clj to run a single test while connecting via SSL with PEM cert

Update CircleCI config:
 - use extra-env to set all the MySQL SSL instance DB related vars (for an RDS instance, currently)
 - adding the rds-combined-ca-bundle.pem certificate to resources/certificates
 - loading that cert bundle from resources directory via env var

Adding to/fixing assertion in connection-spec-test for :ssl