[avro] Support decimal logical type on fixed base (#798)

[29shivam]

Summary

Fixes #798 (part of #726).

Confluent's Avro converter maps both bytes- and fixed-based Avro decimals to a Kafka Connect Decimal logical type whose value is a BigDecimal. The connector already handles that Connect representation end to end — schema validation (ClickHouseWriter.validateDataSchema), conversion (StructToJsonMap), and RowBinary serialization (doWriteColValue) all treat a Connect Decimal as a ClickHouse Decimal(P, S). The already-compatible decimal_bytes fixture confirms the bytes base; this change confirms the fixed base is handled the same way.

Changes:

• Add AvroDecimalLogicalTest — models the Connect Decimal (with connect.fixed.size) that the converter emits for a decimal-on-fixed field, drives it through the convert + write path, and asserts the RowBinary bytes match BinaryStreamUtils.writeDecimal(...). Runs without a live ClickHouse.
• Promote decimal_fixed_logical.json from incompatible/ to compatible/ so the Avro integration test exercises it end to end.
• CHANGELOG entry under 1.4.1 (unreleased).

Verification: the feature test passes locally; the full end-to-end path (Confluent Platform → schema registry → Avro converter → ClickHouse) runs via the integration test in CI.

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG

[29shivam]

@chernser ready for review — this covers the decimal logical-type sub-issue (#798). The Confluent Avro converter already produces a Connect Decimal for both bytes and fixed bases, so the connector handles it end to end; I added a feature test and promoted the fixture to compatible so integration exercises it.