Fixes a bunch of static typing issues in modal.object [CLI-312]

[freider]

• Splits object.py into _object.py with implemenation and object.py with synchronicity wrappers
• Fixes a lot of type errors that surfaced as a result

Describe your changes

• Provide Linear issue reference (e.g. MOD-1234) if available.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Changelog

• Internal refactor of the modal.object module. All entities except Object from that module have now been moved to the modal._object "private" module.

[freider]

I split the work into two commits to make reviewing a bit easier - the first one just renames object.py -> _object.py, and the second one applies all of the actual changes

[erikbern]

🆒

[freider]

@prbot approve

[modal-pr-review-automation]

Approved 👍. @mwaskom will follow-up review this.