feat: Adding settings, utils, write to S3, and writer close

[malteos]

This PR integrates a couple of general changes from the EOT PR (#54):

• Settings variables are loaded from environment variables in settings.py
• Common CLI methods are moved to utils.py
• Read and write to S3 (via fsspec).
• Missing writer.close() statement is added to CLI and example.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.64%. Comparing base (b6f5f02) to head (4e660a3).

Files with missing lines	Patch %	Lines
cdx_toolkit/warc.py	93.10%	2 Missing ⚠️
cdx_toolkit/cli.py	75.00%	1 Missing ⚠️
cdx_toolkit/commoncrawl.py	80.00%	1 Missing ⚠️
cdx_toolkit/utils.py	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   95.78%   95.64%   -0.14%     
==========================================
  Files           7        9       +2     
  Lines         877      918      +41     
==========================================
+ Hits          840      878      +38     
- Misses         37       40       +3     

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