Dont forward datatype extensions beyond the FFI barrier

[teh-cmc]

// NOTE: Do not carry the extension metadata beyond the FFI barrier in order the match the
// data sent by other SDKs.
//
// We've stopped using datatype extensions overall, as they generally have been creating more
// problems than they have solved.
//
// With the addition of `Chunk` and `ChunkMetadata`, it is likely that we will get rid of extension types
// entirely at some point, since it looks like we won't have any use for them anymore.
//
// Doing so will require a more extensive refactoring of the Python SDK though, so until we're absolutely
// certain where we're going, this is a nice, painless and easily reversible solution.

• Fixes Inconsistency in usage of Extension types between python and rust #6606
• Makes main 🟢 again

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