[All] Dynamic protobuf support: runtime descriptors and messages without codegen

[teodordelibasic-db]

Goal

Let users stream without a local .proto file or codegen step: obtain the table's protobuf descriptor at runtime, and build and ingest records dynamically against it.

Two deliverables per SDK

• Descriptor at runtime: build a protobuf descriptor without a compiled .proto file — either from a Unity Catalog table schema, or constructed programmatically in code.
• Dynamic messages: construct and ingest records at runtime against that descriptor, with no statically compiled proto types.

API contract

The canonical way records are supplied dynamically (native dynamic-message objects vs JSON encoded against the descriptor) is decided in the Rust ticket #372. The wrapper SDKs follow that contract, which is why they are blocked by it.

Status

• Rust descriptor generation from UC: shipped (PR feat: Add schema module for UC → proto descriptor conversion  #200; UC to Arrow in PR feat: add new SDK function to convert UC schema to arrow schema #270).
• C/FFI descriptor + JSON encode: in progress (PR [Rust][FFI] Expose UC-schema → protobuf descriptor + record encoding #371).
• Go descriptor generation: in progress (PR [Go] Add dynamic proto descriptor generation from UC schema #256).

Notes

Per-SDK work is tracked in the sub-issues. Supersedes #240, whose per-language descriptor work is folded into them.