Add callback urls back in for NERSC file transfers

[MrCreosote]

Tested in the same way as prior URLs except pointed the callback urls at CI Groups and checked the service logs for requests

also DRYed up manifest generation a little

[codecov]

Codecov Report

Attention: Patch coverage is 23.52941% with 13 lines in your changes missing coverage. Please review.

Project coverage is 80.07%. Comparing base (a053c87) to head (8e2eb03).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
cdmtaskservice/nersc/manager.py	12.50%	7 Missing ⚠️
cdmtaskservice/nersc/remote.py	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   81.21%   80.07%   -1.14%     
==========================================
  Files           9        9              
  Lines         527      537      +10     
==========================================
+ Hits          428      430       +2     
- Misses         99      107       +8     

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[Tianhao-Gu]

LGTM