PYTHON-1357 - Refactor Cursor and CommandCursor

[NoahStapp]

[PYTHON-1357]

Changes in this PR

• Refactor code shared between (Async)Cursor and (Async)CommandCursor into abstracted base classes.
• _AgnosticCursorBase is the top-level base class that is IO-agnostic and shared between both async and sync cursor implementations.
• _AsyncCursorBase and _CursorBase are async and sync-specific base classes that contain code shared between cursor implementations in either the async or sync APIs.
• These changes must not introduce any user-facing changes.
• Two type hints differ between Cursor and CommandCursor: Cursor.cursor_id -> Optional[int] and Cursor.address -> Optional[tuple[str, Any]], while CommandCursor.cursor_id -> int and CommandCursor.address -> Optional[_Address]. Do we count these type hint changes as user-facing, public API changes? Note that _Address = Tuple[str, Optional[int]].
• This is a refactor of identical or nearly identical common code only. Some existing code such as _send_message is similar between the cursor types but has enough differences to be effectively separate implementations.

Test Plan

Run against the existing test suite.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[blink1073]

I think that this is the only class in the hierarchy that needs Generic[_DocumentType] explicitly spelled out