Add BinaryFormatSupport and Row Encoder to arrow-avro Writer

[jecsand838]

Which issue does this PR close?

• Closes [arrow-avro] Add Avro BinaryFormat (Unframed) to writer module #8701.

Rationale for this change

arrow-avro already supports writing Avro Object Container Files (OCF) and framed streaming encodings (e.g. Single-Object Encoding / registry wire formats). However, many systems exchange raw Avro binary datum payloads (i.e. only the Avro record body bytes) while supplying the schema out-of-band (configuration, RPC contract, topic metadata, etc.).

Without first-class support for unframed datum output, users must either:

• accept framing overhead that downstream systems don’t expect, or
• re-implement datum encoding themselves.

This PR adds the missing unframed write path and exposes a row-by-row encoding API to make it easy to embed Avro datums into other transport protocols.

What changes are included in this PR?

• Added AvroBinaryFormat (unframed) as an AvroFormat implementation to emit raw Avro record body bytes (no SOE prefix and no OCF header) and to explicitly reject container-level compression for this format.
• Added RecordEncoder::encode_rows to encode a RecordBatch into a single contiguous buffer while tracking per-row boundaries via appended offsets.
• Introduced a higher-level Encoder + EncodedRows API for row-by-row streaming use cases, providing zero-copy access to individual row slices (via Bytes).
• Updated the writer API to provide build_encoder for stream formats (e.g. SOE) and added row-capacity configuration to better support incremental/streaming workflows.
• Added the bytes crate as a dependency to support efficient buffering and slicing in the row encoder, and adjusted dev-dependencies to support the new tests/docs.

Are these changes tested?

Yes.

This PR adds unit tests that cover:

• single- and multi-column row encoding
• nullable columns
• prefix-based vs. unprefixed row encoding behavior
• empty batch encoding
• appending to existing output buffers and validating offset invariants

Are there any user-facing changes?

Yes, these changes are additive (no breaking public API changes expected).

• New writer format support for unframed Avro binary datum output (AvroBinaryFormat).
• New row-by-row encoding APIs (RecordEncoder::encode_rows, Encoder, EncodedRows) to support zero-copy access to per-row encoded bytes.
• New WriterBuilder functionality (build_encoder + row-capacity configuration) to enable encoder construction without committing to a specific Write sink.

[jecsand838]

@mbrobbel @alamb @scovich @nathaniel-d-ef

Would any of you have bandwidth to review this PR? Much of the diff is comments and tests. I was hoping to get this out in the v58.0.0 release. This is also rather pivotal for the future direction of the arrow-avro Writer, so I'd absolutely love feedback regarding the row-wise Encoder architecture.