API, Core: Add UDF leaf types — Representation and Parameter

[huaxingao]

Add the foundational model classes for UDF metadata

• UdfRepresentation / SQLUdfRepresentation — interfaces for UDF implementation representations
• UdfParameter — interface for function parameters with support for primitive and nested types
• UnknownUdfRepresentation — forward-compatible fallback for unrecognized representation types
• JSON parsers and type utilities for all of the above

[huaxingao]

@flyrain @amogh-jahagirdar @singhpk234 @szehon-ho

Could you please take a look at this PR when you have a moment?

This PR adds the leaf-level UDF model classes (Representation and Parameter) as defined in the UDF spec. I followed the same code pattern as the view implementation (api/.../view/, core/.../view/).
This is the first in a series of PRs to build out UDF metadata support. The next PR will add UdfDefinition and UdfDefinitionVersion on top of these types.