Support for jsonl.gz input in pyserini.encode

[ftvalentini]

Would be nice to add support for reading jsonl.gz files when encoding a corpus with a dense encoder with python -m pyserini.encode, in:

pyserini/pyserini/encode/_base.py

Line 133 in b7e1da3

with open(filename) as f:

Maybe with:

#...
open_handle = gzip.open if filename.endswith(".gz") else open
with open_handle(filename) as f: 
#...

In this way both pyserini.index.lucene and pyserini.encode accept jsonl.gz files as input.