Variables: Support custom sources in internal resolver

[medikoo]

Needed for dashboard plugin variables test refactor

[codecov]

Codecov Report

Merging #10393 (7443cba) into master (352553e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 7443cba differs from pull request most recent head 405a826. Consider uploading reports for the commit 405a826 to get more accurate results

@@           Coverage Diff           @@
##           master   #10393   +/-   ##
=======================================
  Coverage   85.37%   85.37%           
=======================================
  Files         340      340           
  Lines       14004    14005    +1     
=======================================
+ Hits        11956    11957    +1     
  Misses       2048     2048           

Impacted Files	Coverage Δ
lib/configuration/variables/index.js	90.47% <100.00%> (+0.47%)	⬆️

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[pgrzesik]

👍