Support REST Proxy in Direct Connections

Direct Connections allow users to interact with Kafka clusters via the Kafka native protocol. Some users reported having connectivity issues using them on bad internet connections (e.g., https://github.com/confluentinc/vscode/issues/1781).

In addition to the Kafka native protocol, we should support the Kafka REST Proxy in Direct Connections, which seems to be more stable. We might want to limit this to Direct Connections of type Confluent Cloud. This feature will require some work on the sidecar side.