Support for data decoding functions

[vbarua]

During discussion around disallowing the any type in function return values (see PR), the example of

  - name: from_avro
    description: FromAvro
    impls:
      - args:
          - value: binary?
            name: input
          - value: string
            name: avro_schema
        return: any?

came up as an example of when it might be wanted.

In this function, the type of the output any? is derived based on the avro_schema string. Being able to decode binary input like this is a valid usecase, however, we should provide a mechanism to allow this that doesn't rely on an any return type.

In this case, especially, the fact that plan producer can provide the avro_schema means that it has enough information to be able to provide an concrete Substrait type for the output.

[EpsilonPrime]

A similar function is from_substrait. Once decoded the plan can be evaluated to find the schema for the function. It's not something easy to replicate in a functional specification. In PostgreSQL the set returning functions need to specify the schema with an AS clause. That approach may work here.