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 72.57257% with 274 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.64%. Comparing base (2d6562b) to head (e426d71).

Files with missing lines	Patch %	Lines
core/connectors/sdk/src/encoders/avro.rs	72.61%	105 Missing and 7 partials ⚠️
core/connectors/sdk/src/transforms/avro_convert.rs	76.98%	51 Missing and 4 partials ⚠️
core/connectors/sdk/src/decoders/avro.rs	82.63%	43 Missing and 7 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     73.17%   59.64%   -13.54%     
  Complexity      943      943               
=============================================
  Files          1123     1125        +2     
  Lines         97892    89792     -8100     
  Branches      75065    66984     -8081     
=============================================
- Hits          71632    53554    -18078     
- Misses        23671    33621     +9950     
- Partials       2589     2617       +28     

Components	Coverage Δ
Rust Core	55.77% <72.57%> (-18.28%)	⬇️
Java SDK	62.30% <ø> (ø)
C# SDK	69.10% <ø> (-0.31%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.40% <ø> (+0.10%)	⬆️
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	80.65% <0.00%> (-0.34%)	⬇️
core/connectors/sdk/src/transforms/mod.rs	29.62% <0.00%> (-3.71%)	⬇️
core/connectors/sdk/src/lib.rs	56.17% <0.00%> (-4.07%)	⬇️
...ore/connectors/sdk/src/transforms/proto_convert.rs	72.34% <0.00%> (-0.68%)	⬇️
core/connectors/sinks/http_sink/src/lib.rs	85.70% <0.00%> (-0.58%)	⬇️
core/connectors/runtime/src/sink.rs	72.40% <21.42%> (-2.12%)	⬇️
core/connectors/runtime/src/source.rs	68.07% <21.42%> (-1.42%)	⬇️
core/connectors/sdk/src/encoders/proto.rs	70.54% <0.00%> (-1.08%)	⬇️
core/connectors/sdk/src/decoders/avro.rs	82.63% <82.63%> (ø)
... and 2 more

... and 224 files with indirect coverage changes

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