UDF Spec: Consider adding an optional SQL-standard "specific name" to the function definition model

**Background** While reviewing the REST spec PR for list/load function endpoints (#15180), @szehon-ho raised (with SQL-standard input from @srielau) the idea of adding a "specific name" to the UDF function definition model. In the SQL standard, a routine has: a routine name — the user-facing function name, which may be shared across overloads, and a specific name — a unique name identifying a single overload. **Current state** The model already has definition-id, which uniquely identifies one overload within a function (each FunctionDefinition carries its own definition-id). This effectively fills the SQL "specific name" role today. **Proposal to discuss** Add an optional specific-name field on the function definition model as a convenience alongside definition-id, so engines can agree on a stable, human-readable name for a given overload (rather than only the opaque definition-id).