[py-tx] Implementation of IVF faiss indices in PDQ

[b8zhong]

Summary

PDQ now using IVF Faiss for better scalability (as we move away from the old imp)

• Adds PDQSignalTypeIndex2 that automatically switches between flat and IVF indices based on dataset size
• Uses IVF-Faiss for datasets >= 1000 entries (as you said), flat index for less than
• Maintains backward compatibility with existing PDQIndex
• Updates signal.py to use the new index implementation as default (if we're not swapping yet, I'll change it back)

Test Plan

Run the tests....

python3 -m pytest threatexchange/signal_type/tests/test_pdq_signal_type_index2.py -v -W ignore::DeprecationWarning

Passes.. ?