Add support for PanOS XML configuration backup

[pv2b]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

This solves #440 by adding functionality to back up Palo Alto firewall XML configuration files using the HTTPS API.

[codecov-commenter]

Codecov Report

Merging #2360 (cd6031a) into master (8179f48) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2360   +/-   ##
=======================================
  Coverage   62.59%   62.59%           
=======================================
  Files          30       30           
  Lines        1497     1497           
=======================================
  Hits          937      937           
  Misses        560      560           

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[mortzu]

Please process the checklist