feat(connectors): implement Avro payload support with separate encoder, decoder and transform crates

[Tyooughtul]

Which issue does this PR close?

Closes #1846

Rationale

Issue #1846 requested support for Apache Avro as a first-class payload format in connectors, including dedicated StreamEncoder / StreamDecoder implementations and cross-format transformations. This PR delivers that support with schema-driven encoding/decoding and runtime integration.

What changed?

What changed?

Connectors only supported JSON, Raw, Text, Proto and FlatBuffer payloads. Avro data had to be handled as opaque raw bytes without schema validation or format conversion.
So, I added full Avro support across the connector stack, SDK-level AvroStreamEncoder/Decoder with inline/file-based schema loading, a new AvroConvert transform for cross-format conversion, runtime config fields for producers/consumers, and HTTP sink base64 encoding for Avro payloads.

Local Execution

• Passed / not passed
  Passed
• Pre-commit hooks ran / not ran
  Ran

AI Usage

I used codex to check the code I wrote, and it helped me discover some bugs and wrote the pr draft.

[codecov]

Codecov Report

❌ Patch coverage is 45.84585% with 541 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.66%. Comparing base (868ae62) to head (bfe9d51).

Files with missing lines	Patch %	Lines
core/connectors/sdk/src/encoders/avro.rs	45.23%	215 Missing and 9 partials ⚠️
core/connectors/sdk/src/decoders/avro.rs	46.52%	145 Missing and 9 partials ⚠️
core/connectors/sdk/src/transforms/avro_convert.rs	55.64%	102 Missing and 4 partials ⚠️
core/connectors/runtime/src/sink.rs	21.42%	11 Missing ⚠️
core/connectors/runtime/src/source.rs	21.42%	11 Missing ⚠️
core/connectors/sdk/src/encoders/proto.rs	0.00%	11 Missing ⚠️
...ore/connectors/sdk/src/transforms/proto_convert.rs	0.00%	7 Missing ⚠️
core/connectors/sinks/http_sink/src/lib.rs	0.00%	7 Missing ⚠️
core/connectors/sdk/src/lib.rs	0.00%	6 Missing ⚠️
core/connectors/sdk/src/transforms/mod.rs	0.00%	3 Missing ⚠️
... and 1 more

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3141       +/-   ##
=============================================
- Coverage     73.64%   52.66%   -20.98%     
  Complexity      943      943               
=============================================
  Files          1141     1143        +2     
  Lines        100869    91816     -9053     
  Branches      78044    69009     -9035     
=============================================
- Hits          74285    48359    -25926     
- Misses        23928    40983    +17055     
+ Partials       2656     2474      -182     

Components	Coverage Δ
Rust Core	46.47% <45.84%> (-28.14%)	⬇️
Java SDK	62.30% <ø> (ø)
C# SDK	69.11% <ø> (-0.29%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.53% <ø> (ø)
Go SDK	39.41% <ø> (ø)

Files with missing lines	Coverage Δ
core/connectors/runtime/src/configs/connectors.rs	28.18% <ø> (ø)
core/connectors/sdk/src/encoders/flatbuffer.rs	57.61% <0.00%> (-23.38%)	⬇️
core/connectors/sdk/src/transforms/mod.rs	29.62% <0.00%> (-3.71%)	⬇️
core/connectors/sdk/src/lib.rs	55.05% <0.00%> (-5.19%)	⬇️
...ore/connectors/sdk/src/transforms/proto_convert.rs	58.37% <0.00%> (-14.65%)	⬇️
core/connectors/sinks/http_sink/src/lib.rs	57.35% <0.00%> (-28.93%)	⬇️
core/connectors/runtime/src/sink.rs	72.40% <21.42%> (-2.12%)	⬇️
core/connectors/runtime/src/source.rs	66.90% <21.42%> (-2.84%)	⬇️
core/connectors/sdk/src/encoders/proto.rs	43.42% <0.00%> (-28.21%)	⬇️
core/connectors/sdk/src/transforms/avro_convert.rs	55.64% <55.64%> (ø)
... and 2 more

... and 362 files with indirect coverage changes

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