This commit added the `ssl_enabled` and `ssl_client_authentication` configs. It also deprecates the following settings to comply with Logstash's SSL naming convention:
- `ssl` was deprecated in favor of `ssl_enabled`
- `ssl_verify_mode` was deprecated  in favor of `ssl_client_authentication`

In addition to the new settings, it also changes:
- Using `ssl_verify_mode` should keep the current behavior as it is. Once upgraded to `ssl_client_authentication`, it will validate and reject configurations with `ssl_certificate_authorities` set and `ssl_client_authentication = > none`.  The current `ssl_verify_mode` behavior is to silently ignore the `none` value and use `force_peer/required`.
- Fixed `ssl_peer_metadata` when `ssl_enabled => false`
- Added tests for the new and existing `SslContextBuilder` methods.