Add options --cert and --client-cert to pip-sync

[atugushev]

Closes #713.

Changelog-friendly one-liner: Add options --cert and --client-cert to pip-sync

Contributor checklist

• Provided the tests for the changes.
• Requested a review from another contributor.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[codecov]

Codecov Report

Merging #798 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #798      +/-   ##
==========================================
+ Coverage   98.06%   98.07%   +<.01%     
==========================================
  Files          34       34              
  Lines        1965     1971       +6     
  Branches      257      259       +2     
==========================================
+ Hits         1927     1933       +6     
  Misses         26       26              
  Partials       12       12

Impacted Files	Coverage Δ
tests/test_cli_sync.py	100% <ø> (ø)	⬆️
piptools/scripts/sync.py	100% <100%> (ø)	⬆️

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[atugushev]

@blueyed thanks for reviewing this!