SSL/TLS support for backend connections

[renecannao]

Backend connections currently do not use SSL/TLS. For production:

• MySQL: mysql_ssl_set() before connect
• PostgreSQL: sslmode=require in connection string
• Certificate verification
• Client certificate authentication

[renecannao]

Implemented in cc1aa85: SSL/TLS support for backend connections. Added ssl_mode/ssl_ca/ssl_cert/ssl_key to BackendConfig, applied in ConnectionPool, MySQLRemoteExecutor, and PgSQLRemoteExecutor. URL query param parsing supports all SSL fields. 8 new tests added.