[Schemas] Use configured schema service in Avro processors

[hariso]

Part of #1560.

With schema support in Conduit, we come to the questions:

• Should the Avro processors be able to use Conduit's schema service?
• If yes, how?

[hariso]

Comment by @lovromazgon:

Not for this PR, but I'll mention it before I forget. I realized that having a builtin schema registry could create the expectation that the avro.decode and avro.encode processors would be able to use the internal schema registry. It would be a bit strange if we had to configure Conduit to point to a schema registry and then again supply the URL to the processors separately. So ideally the processors should use this service to create and get schemas. Not sure if this was included in the design or not.

https://github.com/ConduitIO/conduit/pull/1605#discussion_r1635023006