Add option to specify a branch to serve on

[y-popov]

No description provided.

[y-popov]

Add option when starting core service to use a new branch, so web changes can be tested

[codecov]

Codecov Report

Attention: Patch coverage is 24.39024% with 31 lines in your changes are missing coverage. Please review.

Project coverage is 59.00%. Comparing base (d8e41f3) to head (d4252c4).

Files	Patch %	Lines
softpack_core/schemas/environment.py	5.88%	16 Missing ⚠️
softpack_core/artifacts.py	36.84%	11 Missing and 1 partial ⚠️
softpack_core/service.py	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #51      +/-   ##
===========================================
- Coverage    59.85%   59.00%   -0.86%     
===========================================
  Files           17       17              
  Lines          944      961      +17     
  Branches       175      177       +2     
===========================================
+ Hits           565      567       +2     
- Misses         372      385      +13     
- Partials         7        9       +2     

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[mjkw31]

LGTM.