feat(proto): Add protobuf support for user-defined-function extension

[rtpsw]

Currently, Ibis has vectorized UDF nodes that are not handled by Ibis-Substrait (see ibis-project/ibis-substrait#236). This issue is specific for the protobuf changes needed.

[westonpace]

This issue is somewhat vague (is this for embedding UDF implementation into the plan? For calling pre-registered UDFs in the consumer? etc?) Could you clarify? I know we've all done some work in this area over the last six months. Alternatively, we could close this and reopen issues / discussion at a later time.

[rtpsw]

This is for embedding vector UDF implementations - using the func member. See test_udf.py in Arrow for the scalar (rather than vector) UDF is currently implemented in a Substrait consumer.

AFAIK, this issue can be held for the time being. @icexelloss could bear on prioritization.