Avatar resolver

[core23]

Closes #11

Subject

Add API for a custom avatar resolver service.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@9f2e6ef). Click here to learn what that means.
The diff coverage is 58.62%.

@@            Coverage Diff            @@
##             master      #12   +/-   ##
=========================================
  Coverage          ?   74.28%           
  Complexity        ?      239           
=========================================
  Files             ?       27           
  Lines             ?      731           
  Branches          ?        0           
=========================================
  Hits              ?      543           
  Misses            ?      188           
  Partials          ?        0

Impacted Files	Coverage Δ	Complexity Δ
src/Twig/ImpersonateExtension.php	0% <ø> (ø)	6 <0> (?)
src/Avatar/StaticAvatarResolver.php	0% <0%> (ø)	3 <3> (?)
src/Twig/AvatarExtension.php	0% <0%> (ø)	4 <4> (?)
src/Twig/GlobalVariables.php	50% <100%> (ø)	2 <1> (?)
.../DependencyInjection/NucleosUserAdminExtension.php	91.93% <100%> (ø)	16 <2> (?)
src/DependencyInjection/Configuration.php	100% <100%> (ø)	2 <1> (?)

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