Implement new Pregel._defaults() method to match Python implementation

[Andrew Nguonly (andrewnguonly)]

Summary

_defaults() is a new method in the Pregel class (Python implementation) that the updated invoke() and stream() methods depend on.

Reference: https://github.com/langchain-ai/langgraph/blob/main/langgraph/pregel/__init__.py#L558

Implementation

1. Create new _defaults() method.
2. Create new streamChannelsAsIs getter. _defaults() depends on this.
3. Create new All (*) string literal type. _defaults() depends on this. However, this feature was added as part of another feature unrelated to _defaults(). See this PR: Implement interrupt_before=* and interrupt_after=* langgraph#367.

Next Steps

1. Update Pregel invoke() and stream() methods.
2. Update Graph APIs and remaining Pregel state APIs.