feat(cuDF): Support function signature retrieval from registry

[pramodsatya]

Summary

• Expose the cuDF scalar function registry so Velox can surface fully-qualified function names together with their exec::FunctionSignature metadata.
• Add utilities that convert the cuDF scalar and aggregation registries into FunctionSignatureMap / AggregateFunctionSignatureMap, reusing the existing signature builders to preserve type variables, intermediate types, and varargs info.
• Make these maps available to higher layers (e.g., Presto native sidecar) so external engines can be made aware of GPU function capabilities.

Motivation

Presto’s native sidecar plugin needs to enumerate function signatures to manage cuDF functions, this is achieved via the /v1/functions endpoint on the Presto C++ sidecar. This Velox change exposes cuDF functions metadata to Presto via the sidecar, allowing for seamless integration of cuDF functions into Presto. Making the coordinator aware of function signatures supported for cuDF execution enables intelligent execution-aware planning and unlocks fail-fast behavior.

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[pramodsatya]

@majetideepak, @devavret, could you please help review this change?

[github-actions]

Build Impact Analysis

Full build recommended. Files outside the dependency graph changed:

• velox/experimental/cudf/exec/CudfHashAggregation.cpp
• velox/experimental/cudf/exec/CudfHashAggregation.h
• velox/experimental/cudf/expression/ExpressionEvaluator.cpp
• velox/experimental/cudf/expression/ExpressionEvaluator.h

These directories are not fully covered by the dependency graph. A full build is the safest option.

cmake --build _build/release

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