Add support for storing the "local" pool on Azure

[neolynx]

Replaces #1074

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Checklist

• [ ] unit-test added (if change is algorithm)
• [ ] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated
• [ ] author name in AUTHORS

[neolynx]

might have rebase errors in azure/public.go since prefix was introduced

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 67.30245% with 120 lines in your changes are missing coverage. Please review.

Project coverage is 74.67%. Comparing base (8d09c20) to head (7b1917f). Report is 1 commits behind head on master.

Files	Patch %	Lines
azure/package_pool.go	64.33%	34 Missing and 17 partials :warning:
azure/azure.go	76.82%	13 Missing and 6 partials :warning:
api/mirror.go	25.00%	9 Missing and 3 partials :warning:
cmd/mirror_update.go	25.00%	9 Missing and 3 partials :warning:
utils/config.go	62.50%	9 Missing and 3 partials :warning:
files/public.go	47.61%	8 Missing and 3 partials :warning:
context/context.go	83.33%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1283      +/-   ##
==========================================
- Coverage   74.79%   74.67%   -0.12%     
==========================================
  Files         144      146       +2     
  Lines       16256    16489     +233     
==========================================
+ Hits        12158    12313     +155     
- Misses       3156     3205      +49     
- Partials      942      971      +29     

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[neolynx]

Continued in https://github.com/aptly-dev/aptly/pull/1074