Transform Avro records into JSON

[jrkinley]

Provide the ability to decode Avro records received by the agent and transform them into JSON before writing them to the synchronised (local) topic.

The agent can fetch the schema in one of two ways:

1. Directly from the source clusters schema registry over HTTPS, or
2. From a local schema registry (e.g. the native Redpanda registry). Assuming that the source schema registry's topic (e.g. _schemas) has been replicated and is used as the backing store for the local schema registry