[Rust] Add support to fetch schema from UC for dynamic proto

[andrijast-db]

What changes are proposed in this pull request?

Follow-up to #465, closing #521. Adds the ability to fetch a table's schema from Unity Catalog and use it as a dynamic-proto descriptor, so the descriptor no longer has to be built column-by-column in code.

New public API (all additive):

• uc_schema module — fetch_message_descriptor(...) mints an OAuth token, reads GET /api/2.1/unity-catalog/tables/{name}, and resolves it to a MessageDescriptor; fetch_table_schema(...) returns the raw schema.
• ZerobusSdk::fetch_message_descriptor(table, client_id, client_secret) — wrapper using the SDK's unity_catalog_url.
• ZerobusError::SchemaFetchError { message, retryable }.

let descriptor = sdk.fetch_message_descriptor("catalog.schema.table", client_id, client_secret).await?;
let stream = sdk.stream_builder().table("catalog.schema.table")
.oauth(client_id, client_secret).dynamic_proto(descriptor).build().await?;

The fetch is a separate step feeding the existing .dynamic_proto(...) — rather than a new builder method that fetches inside build() — so .dynamic_proto() stays the single format selector, network I/O stays visible, and one descriptor can be reused across streams. It mints a plain all-apis token (the ingestion token DefaultTokenFactory produces is rejected by the UC REST API), bounds response body size, classifies transport/5xx/429 as retryable, and rejects credential-bearing URLs. No FFI/wrapper changes.

How is this tested?

Full cargo test --workspace passing; clippy (--all --all-features -D warnings) and fmt clean. Unit tests cover validation, endpoint/URL handling, and descriptor conversion; 20 integration tests run against a loopback HTTP mock covering the happy path, request shapes, and error classification (4xx/5xx/429, unparseable/oversized body, dropped connection). Not tested against a live workspace.