[Rust][FFI] Add Avro C ABI (avro feature)

[irinatomic-db]

PR stack

• avro/proto-surface [MERGED]
  • avro/purego
  • avro/rust-core
    • avro/ffi-c-abi <- this PR
      • avro/go
      • avro/cpp
      • avro/dotnet
    • avro/java
    • avro/python
    • avro/typescript

What changes are proposed in this pull request?

Exposes the Rust SDK's Avro support across the C FFI, behind the off-by-default avro feature (Beta). Additive only — existing C signatures and CStreamConfigurationOptions are byte-for-byte unchanged, so Go/C++/.NET keep building against the current ABI.

• ffi/Cargo.toml: new avro feature forwarding to databricks-zerobus-ingest-sdk/avro.
• ffi/cbindgen.toml: [defines] "feature = avro" = "ZEROBUS_AVRO", so the avro declarations are emitted in zerobus.h under #if defined(ZEROBUS_AVRO). The header is identical with or without the feature (freshness check stays green).
• ffi/src/stream.rs:
  • New create fns zerobus_sdk_create_avro_stream (+ _async / _with_headers_provider _with_headers_provider_async), taking avro_schema_json as a null-terminated C string in the descriptor slot.
  • New ingest fns zerobus_stream_ingest_avro_record[s] (+ _async / _nowait), building EncodedRecord::Avro (raw pre-encoded datums, mirroring the proto path).
  • build_stream_from_parts gained an avro_schema_json param (forces RecordType::Avro → .avro(schema)); the EncodedRecord→C-array match gained a gated Avro arm.
• Regenerated ffi/zerobus.h (avro surface under #if defined(ZEROBUS_AVRO)).
• ffi/NEXT_CHANGELOG.md entry; ci-rust.yml gains an --features avro build/test step.

Consumers build the FFI with --features avro and compile with -DZEROBUS_AVRO. Ephemeral streams only; server support is pending.

Once the avro gate is removed the new CI job will be removed as well.

How is this tested?

cd rust

# With the feature
cargo build -p zerobus-ffi --features avro
cargo test  -p zerobus-ffi --features avro
cargo clippy -p zerobus-ffi --features avro --all-targets

# Default (feature off) — no regressions
cargo build -p zerobus-ffi
cargo test  -p zerobus-ffi

# Whole workspace builds/tests
cargo build --workspace
cargo test  --workspace --no-run

cargo fmt --all --check

# Header stability: additive-only and identical with/without the feature,
# so the zerobus.h freshness check stays green.
cargo build -p zerobus-ffi && cp ffi/zerobus.h /tmp/h.h
cargo build -p zerobus-ffi --features avro && diff /tmp/h.h ffi/zerobus.h
git diff ffi/zerobus.h    # purely additive