Implement /resend-pending-builds endpoint

[sersorrel]

This endpoint finds all "pending" builds (not yet successful/failed) and resends them to the builder.

Additionally, queue environments for creation even when the builder is unavailable (because it might come back up later and ask for them).

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (16e49df) 65.29% compared to head (e2819fa) 61.83%.
Report is 11 commits behind head on develop.

Files	Patch %	Lines
softpack_core/service.py	26.31%	14 Missing ⚠️
softpack_core/schemas/environment.py	13.33%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #48      +/-   ##
===========================================
- Coverage    65.29%   61.83%   -3.46%     
===========================================
  Files           16       17       +1     
  Lines          775      896     +121     
  Branches       131      163      +32     
===========================================
+ Hits           506      554      +48     
- Misses         262      335      +73     
  Partials         7        7              

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[mjkw31]

LGTM.