feat: install Ubuntu Pro Client in LXD instance

[linostar]

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Intsall Ubuntu Pro Client inside the managed LXD instance, which is necessary for all Pro-related commands such as attach, enable, etc.

[codecov]

Codecov Report

Attention: Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/pro-sources@c511987). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
craft_providers/lxd/lxc.py	95.34%	2 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##             feature/pro-sources     #664   +/-   ##
======================================================
  Coverage                       ?   98.77%           
======================================================
  Files                          ?       55           
  Lines                          ?     4576           
  Branches                       ?        0           
======================================================
  Hits                           ?     4520           
  Misses                         ?       56           
  Partials                       ?        0           

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[linostar]

@canonical/starcraft-reviewers Ready for review.

[mr-cal]

Looks good overall. Can you add test cases for the missing code coverage notices?

[tigarmo]

@mr-cal I think it's ok to address the coverage issues later on, when this feature branch is considered for merging to main

[linostar]

@tigarmo This is ready for merge now.