[py-tx] updated the covaraince of TUpdateRecordValue to allow for mor…

[Mackay-Fisher]

This PR addresses the t.any type in issue #1255 and helps to impose stronger typing in SignalExchangeAPIWithSimpleUpdates by replacing t.Any with state.TFetchedSignalMetadata and making state.TUpdateRecordValue covariant.

Changes:
Replaced t.Any with state.TFetchedSignalMetadata in SignalExchangeAPIWithSimpleUpdates to enhance type specificity and ensure compatibility with the expected metadata structure.

Made state.TUpdateRecordValue Covariant to resolve iterator conflicts in fetch_iter usage across multiple files. The updated covariance improves flexibility and compatibility in the FetchDelta data class, enabling fetch_iter to handle both states.TFetchedSignalMetadata and state.TUpdateRecordValue as well as pre-existing subclass iterator changes.

Effected File List: Cli_state.py, Signal_exchange.py, Fb_threatexchnage.py, File_api.py, Static_sample.py, Stop_ncii_api.py, Techagainstterrorism.py, Test_state.py.

Testing:
Ran all of the type checking with “python -m mypy threatexchange”, then did further testing with
mypy --check-untyped-defs for each affected file in the above effect file list.

When running the specific unit tests, all related tests pass with no changes in functionality; however, test cases in the hashing library fail, which is consistent with the main branch before these changes.

Related Tests:
threatexchange/exchanges/tests/test_state.py
threatexchange/exchanges/tests/test_state_compatibility.py

[Dcallies]

That was simpler than I thought it was going to be, but the checks pass and so I accepts. Nice work!