Split converter.py into submodules

[jmaeagle99]

What was changed

• Broke up the monolithic temporalio/converter.py (~2,100 lines) into a proper Python package under temporalio/converter/ with the following submodules:
  • _serialization_context.py — SerializationContext, WorkflowSerializationContext, ActivitySerializationContext, WithSerializationContext
  • _payload_converter.py — PayloadConverter, EncodingPayloadConverter, CompositePayloadConverter, DefaultPayloadConverter, and all concrete encoding implementations (BinaryNull, BinaryPlain, JSONProto, BinaryProto, JSONPlain, JSONTypeConverter)
  • _payload_limits.py — PayloadLimitsConfig, PayloadSizeWarning, _PayloadSizeError, _ServerPayloadErrorLimits
  • _payload_codec.py — PayloadCodec and _apply_to_failure_payloads (moved out of DataConverter to break a potential circular dependency)
  • _failure_converter.py — FailureConverter, DefaultFailureConverter, DefaultFailureConverterWithEncodedAttributes
  • _data_converter.py — DataConverter and default()
  • _search_attributes.py — all search attribute encode/decode utilities
• __init__.py re-exports all public names and handles module-level initialization (DataConverter.default, PayloadConverter.default, FailureConverter.default), preserving full backward compatibility — no call sites outside the package required changes
• __all__ defined in __init__.py to suppress F401 linter warnings and make the public API explicit; private names (_PayloadSizeError, etc.) are not re-exported but remain accessible via their submodules
• Internal callers in temporalio/worker/ and temporalio/client.py updated to reference private types directly from their submodule (e.g. temporalio.converter._payload_limits._ServerPayloadErrorLimits) rather than through the package namespace
• Both submodule loggers use getLogger("temporalio.converter") to preserve the original log namespace

Why?

Separate the concerns of each composite area and allow other changes (such as #1341) to not introduce circular dependencies.

Checklist

1. How was this tested: Existing tests
2. Any docs updates needed? No

[tconley1428]

Thanks for doing this.