Implement attrs-based payload encoding helpers

[sirosen]

This PR is a proposal for a new impelmentation of payload definitions for primary use in SDK v4 and for any new payload classes under v3.

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This first implementation introduces a new subpackage, globus_sdk.payload which is integrated into the transport layer's request encoding logic.

globus_sdk.payload.Payload defines a base class for @attrs.defined classes, providing support for an extra parameter for what we have historically named addtional_fields.

Payload also gives us a type for usage in type hints for the transport layer and base client, and provides an asdict method which handles any specialized pre-transport encoding steps. For the base this is the handling of extra, but subclasses can extend it if necessary.

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📚 Documentation preview 📚: https://globus-sdk-python--935.org.readthedocs.build/en/935/