feat: add query-scoped extension import handlers

[aunjgr]

What this PR does

Add a converter-scoped C++ extension handler for user-defined types, typed NULLs, literals, scalar functions and aggregates. Preserve resolved extension identities, full type/function parameters and declared result types for embedding consumers, without adding numeric coercion or global registrations.

Why

Embedded Sirius needs to preserve MatrixOne's approved exact-decimal semantics rather than let generic DuckDB decimal binding derive different types. This supplies the import boundary; the numeric implementation and capability advertisement remain in Sirius.

Refs matrixorigin/matrixone#28968 and matrixorigin/matrixone#28966. Numeric contract approved in matrixorigin/matrixone#29449, document revision 42a89f09a1d168d02b9583cb3ea7b4de6dbb5634.

Compatibility and scope

• Targets feature/26155-substrait-anti-joins, the compatible branch pinned by Sirius at b131fd6659fe5fa0c6e39c52059e29ac903b2d59. Importer main has diverged; this PR deliberately does not synchronize dependencies or include unrelated commits.
• Existing constructor and ordinary imports remain available. Consumers opt into the handler through the new overload; no global extension state is installed.
• Duplicate anchors and unresolved URNs fail under opt-in import. Unknown user types/literals fail explicitly. Standard default import behavior is unchanged.
• The importer owns a shared handler reference through conversion. C++ consumers must be rebuilt; no binary class-layout compatibility is claimed. No C ABI or numeric capability version is changed.

Verification

Built against Sirius's pinned DuckDB using its frozen mo Pixi environment. Sirius object/archive build passes. Direct C++ consumer test passes 60 assertions covering complete type parameters, typed NULL, payload identity, full scalar/aggregate signatures, ordinary-import controls and malformed/unhandled extensions. New tests are registered with the importer C++ test target. No Docker image build and no change to DuckDB itself.