[VOTE][REST-SPEC] Add list and load function endpoints

[huaxin gao]

Hi All,

I propose adding REST catalog endpoints for listing and loading functions,
following the Iceberg UDF spec.

New endpoints:

   - GET /v1/{prefix}/namespaces/{namespace}/functions — list functions in
   a namespace
   - GET /v1/{prefix}/namespaces/{namespace}/functions/{function} — load a
   function

ListFunctionsResponse returns FunctionIdentifier objects (namespace +
name), consistent with listTables. LoadFunctionResult returns inline UDF
metadata and optional metadata-location, following the
LoadTableResult/LoadViewResult pattern. Read-only; CRUD endpoints will
follow in a separate proposal.

For details please check:

   - Spec PR: https://github.com/apache/iceberg/pull/15180

Please vote in the next 72 hours:

[ ] +1 Add these changes to the spec
[ ] +0
[ ] -1 I have questions and/or concerns

Best,
Huaxin