Camel Avro RPC extension: Investigate `GenericDatumReaderSubstitute` and refactor it, if it makes sense

`Avro-rpc ` extension contain substitution `GenericDatumReaderSubstitute`.
Reason for the substitution was not clear during creation. See PR [summary](https://github.com/apache/camel-quarkus/pull/2025)

There is a possible explanation in the [comment](https://github.com/apache/camel-quarkus/pull/2025#pullrequestreview-537591608). (thanks @ppalaga !)

It is possible, that after finding real reason of requirement of this substitution, it could be refactored to much easier (also correct and more readable) solution.