[py-tx] Implement Tech Against Terrorism Hash List API

[Dcallies]

Looks like TAT add a hash exchange API, we should add a reference client, here's the documentation: https://www.terrorismanalytics.org/docs/hash-list-v1

A good 1:1 comparison for how to do this is from StopNCII:

If you want to give a crack at it, you can follow the same approach we did for StopNCII

1. Add a reference version for a client for the TAT hash api. Here’s StopNCII’s: https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/exchanges/clients/stopncii/api.py
   1. We should add tests
2. Add an “Exchange” interface implementation, which tells the library how to download a copy of all hashes. Here’s StopNCII’s: https://github.com/facebook/ThreatExchange/blob/main/python-threatexchange/threatexchange/exchanges/impl/stop_ncii_api.py
   1. You can test to see it works correctly using the tx commandline interface to download and then match signals
3. Potentially add implementations for "SHA256", "SHA512", cryptographic types, and a generic "File" content type