Added custom package orderer and fixed package orderer sort logic

[isohedronpipeline]

This carries on the work originally started by @pmolodo with the following PRs:
#355
#413

This change primarily fixes the issue with package orderers not applying to variants, but also adds a new CustomPackageOrder that can be used to configure a specific ranked list of versions.

[codecov]

Codecov Report

Attention: Patch coverage is 91.69329% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 58.67%. Comparing base (153bd87) to head (bbaf290).
Report is 1 commits behind head on main.

❗ Current head bbaf290 differs from pull request most recent head 3a2e3db. Consider uploading reports for the commit 3a2e3db to get more accurate results

Files	Patch %	Lines
src/rez/package_order.py	91.44%	14 Missing and 9 partials ⚠️
src/rez/utils/yaml.py	75.00%	2 Missing ⚠️
src/rez/cli/config.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1680      +/-   ##
==========================================
+ Coverage   58.03%   58.67%   +0.63%     
==========================================
  Files         127      127              
  Lines       17069    17240     +171     
  Branches     3496     3528      +32     
==========================================
+ Hits         9906    10115     +209     
+ Misses       6496     6459      -37     
+ Partials      667      666       -1     

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[isohedronpipeline]

Closed in lieu of #1684