[Java] Dynamic protobuf: runtime descriptor + dynamic message ingestion

[teodordelibasic-db]

Part of #99. Blocked by #372 (the Rust ticket sets the cross-SDK API contract).

Port the dynamic protobuf path to the Java SDK so a user can stream without a .proto file or codegen step, following the API contract decided in #372:

• Build a protobuf descriptor from a Unity Catalog table schema at runtime (mirrors the Rust schema module, PR feat: Add schema module for UC → proto descriptor conversion  #200)
• Provide an ergonomic in-code descriptor builder as an alternative source (no UC, no .proto)
• Construct and ingest records dynamically against that descriptor, without statically compiled proto types
• Example, tests, changelog entry, docs

Note: a C/FFI encode path is being added in PR #371 and may be reusable here.