[feat] Add new build system backend for custom builds

[vkarak]

Fixes #2473

[ekouts]

lgtm

[teojgo]

Since you are touching the buildsystems, can you change the remaining string formatting which uses % to f=strings?

[codecov-commenter]

Codecov Report

Merging #2494 (21a76cb) into master (098ff00) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 21a76cb differs from pull request most recent head ea457f5. Consider uploading reports for the commit ea457f5 to get more accurate results

@@           Coverage Diff           @@
##           master    #2494   +/-   ##
=======================================
  Coverage   85.79%   85.79%           
=======================================
  Files          57       57           
  Lines       10686    10690    +4     
=======================================
+ Hits         9168     9172    +4     
  Misses       1518     1518           

Impacted Files	Coverage Δ
reframe/core/buildsystems.py	96.26% <100.00%> (+0.04%)	⬆️

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