feat(datafusion): support REST Catalog CREATE FUNCTION

[JingsongLi]

Summary

Add persistent CREATE FUNCTION support for SQL scalar functions in Paimon REST Catalogs. The implementation uses the existing REST SQL function expansion path to validate a candidate definition before persisting it, so newly created functions follow the same execution semantics as catalog functions read from REST.

Changes

• Add the Java-compatible create-function REST request, catalog API, POST endpoint, and create-specific 404/409/501 error mapping.
• Support bare, two-part, and three-part creation targets, named nullable parameters, a single nullable return value, and scalar RETURN expressions.
• Align the recommended SQL syntax with Databricks: LANGUAGE SQL is optional with SQL as the default, and determinism is inferred from the planned expression instead of requiring IMMUTABLE; explicit IMMUTABLE remains accepted for compatibility.
• Add token-level adapters for IF NOT EXISTS and OR ALTER, including comments and quoted identifiers, without rewriting function bodies.
• Validate candidate functions before POST using typed placeholders, owning-catalog/database resolution, REST dependency expansion, logical and physical DataFusion planning, recursion detection, volatility checks, and scalar-only restrictions.
• Document supported syntax, resolution rules, validation behavior, and unsupported function forms.

Testing

• cargo test -p paimon --test rest_object_models_test
• cargo test -p paimon --test rest_api_test --test rest_catalog_test
• cargo test -p paimon-datafusion --lib persistent_rest_catalog_function_
• cargo clippy -p paimon -p paimon-datafusion --all-targets -- -D warnings
• cargo fmt --all -- --check and git diff --check

Notes

Persistent function creation is implemented by REST Catalog; other catalog implementations return Unsupported by default. Aggregate, table, multi-return, non-SQL, Stable/Volatile, lambda/file, temporary, alter, and drop function forms remain out of scope.

A full local paimon-datafusion --lib run passed 260 tests; seven existing table-scan tests require pre-populated partitioned_log_table fixtures under /tmp/paimon-warehouse, which are not present in this environment.

[leaves12138]

I found one blocking issue around complex argument/return types. The existing targeted suites and formatting checks pass, but focused regression cases with ARRAY<BIGINT> reproduce failures before the function can be created or executed.

[leaves12138]

Re-reviewed b0dbc50. The shared recursive SQL type renderer now handles both CREATE FUNCTION validation placeholders and REST SQL function return casts. The added ARRAY regressions pass, and I also verified MAP and ROW argument signatures locally. The persistent CREATE FUNCTION suite, REST SQL function suite, and formatting check all pass. LGTM.