feat: add sentence transformers support

[besaleli]

• Add support for sentence transformers
• Add mean_pool tensor operation

[codspeed-hq]

CodSpeed Performance Report

Merging #45 will not alter performance

_{Comparing 38-add-sentence-transformers-support (8b9a596) with main (3b4a68d)}

Summary

✅ 20 untouched

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.26829% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
encoderfile/src/transport/mcp/mod.rs	0.00%	9 Missing ⚠️
encoderfile/src/transforms/engine.rs	94.25%	5 Missing ⚠️
encoderfile/src/runtime/transform.rs	0.00%	3 Missing ⚠️
encoderfile/src/cli.rs	0.00%	2 Missing ⚠️
encoderfile/src/transport/grpc/mod.rs	0.00%	2 Missing ⚠️
encoderfile/src/transforms/tensor/mod.rs	96.42%	1 Missing ⚠️

Files with missing lines	Coverage Δ
encoderfile/src/common/model_type.rs	100.00% <100.00%> (ø)
encoderfile/src/common/sentence_embedding.rs	100.00% <100.00%> (ø)
encoderfile/src/inference/embedding.rs	100.00% <100.00%> (ø)
encoderfile/src/inference/sentence_embedding.rs	100.00% <100.00%> (ø)
...coderfile/src/inference/sequence_classification.rs	100.00% <100.00%> (ø)
encoderfile/src/inference/token_classification.rs	96.55% <100.00%> (-0.42%)	⬇️
encoderfile/src/runtime/state.rs	23.07% <100.00%> (ø)
encoderfile/src/services/model_metadata.rs	100.00% <ø> (ø)
encoderfile/src/services/sentence_embedding.rs	100.00% <100.00%> (ø)
encoderfile/src/test_utils/mod.rs	100.00% <100.00%> (ø)
... and 7 more

... and 1 file with indirect coverage changes

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