This commit added the following SSL settings:
- `ssl_enabled`: Enable/disable SSL
- `ssl_keystore_path`: JKS keystore path
- `ssl_keystore_password`: JKS keystore password
- `ssl_client_authentication`: Controls the server’s behavior in regard to requesting a certificate from client connections
   
To comply with Logstash's SSL naming convention, It also reviewed and deprecated the following SSL settings:
- `ssl` in favor of `ssl_enabled`
- `ssl_verify_mode` in favor of `ssl_client_authentication`
- `keystore` in favor of `ssl_keystore_path`
- `keystore_password` in favor of `ssl_keystore_password`

Other change:
- Added tests for new and existing `SslSimpleBuilder` methods.
- Moved SSL context builder logic from `SslHandlerProvider` to `SslSimpleBuilder`