Refactoring to remove duplicates in pyserini.encode and pyserini.search.faiss

[lintool]

Addresses #1728 which will in turn contributes to #2006

[lintool]

• I've reconciled the two different versions of QueryEncoder and the two versions of AutoQueryEncoder; retaining only the version in pyserini.encode.
• Also de-dupped the encoder implementations in pyserini.search.faiss, folding into pyserini.encode as appropriate.
• Only remaining encoders in pyserini.search.faiss._searcher.py are ClipQueryEncoder and OpenAIQueryEncoder, which I'll refactor in a subsequent pass.

This closes #2009

Running tests right now. After confirming they pass, will be ready to merge.

[lintool]

• Successfully passed all integrations, core and optional.
• Successfully ran all below:

[+] nohup ./run-tct2-encoded.sh >& logs/log.tct2-encoded &
[+] nohup ./run-tct2-otf.sh >& logs/log.tct2-otf &
[+] nohup ./run-tct-encoded.sh >& logs/log.tct-encoded &
[+] nohup ./run-tct-otf.sh >& logs/log.tct-otf &
[+] nohup ./run-dpr-encoded.sh >& logs/log.dpr-encoded &
[+] nohup ./run-dpr-otf.sh >& logs/log.dpr-otf &
[+] nohup ./run-ance.sh >& logs/log.ance &
[+] nohup ./run-bpr.sh >& logs/log.bpr &
[+] nohup ./run-distillbert-kd.sh >& logs/log.distillbert-kd &
[+] nohup ./run-distillbert-tasb.sh >& logs/log.distillbert-tasb &
[+] nohup ./run-dkrr.sh >& logs/log.dkrr &
[+] nohup ./run-sbert.sh >& logs/log.sbert &

• Wrapping up all regressions with prebuilt indexes.

[lintool]

All regressions with prebuilt indexes completed successfully as well. Merging.