feat: Add functionality to pass custom serializer

[davidberenstein1957]

Introduce BaseEventSerializer as an abstract base class

This refactoring introduces a new BaseEventSerializer abstract base class that provides a common interface and shared functionality for JSON serialization. Key changes include:

• Creating an abstract base class with common serialization methods

• Moving shared serialization logic to the base class

• Adding an abstract default method for object serialization

• Implementing safe integer range checking

• Updating references in other files to use the new base class

• [ ] documentation with explanation

• [ ] JS SDK

Greptile Summary

Disclaimer: Experimental PR review

Introduces a new BaseEventSerializer abstract base class to standardize JSON serialization across the codebase, enabling custom serializer implementations while maintaining existing functionality.

• Missing initialization of _serializer field in LangfuseClient.__init__ in langfuse/request.py needs to be added to prevent runtime errors
• IngestionConsumer._serializer defined as class variable could cause shared state issues with multiple instances
• No constructor parameter in IngestionConsumer to configure custom serializers without subclassing
• Added abstract BaseEventSerializer with required default() method in langfuse/serializer.py
• Test case for custom serializer extension needs pandas dependency added to test requirements

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[hassiebp]

Thanks again for your contribution! Closing this PR as stale / no longer mergeable in Python SDK v3. If you have specific requirements for a separate serializer, please open a Github issue for us to conduct the discussion there