lakefs superuser command for adding more admin users

[nopcoder]

lakefs superuser - create an additional admin user

Check that Admin group exists and add a new created user to the Admins group

[codecov-io]

Codecov Report

Merging #864 into master will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #864      +/-   ##
==========================================
- Coverage   45.14%   45.09%   -0.05%     
==========================================
  Files         140      140              
  Lines       10855    10862       +7     
==========================================
- Hits         4900     4898       -2     
- Misses       5324     5332       +8     
- Partials      631      632       +1     

Impacted Files	Coverage Δ
auth/setup.go	0.00% <0.00%> (ø)
catalog/cataloger_create_branch.go	84.90% <0.00%> (-3.78%)	⬇️
catalog/cataloger_create_repository.go	59.25% <0.00%> (-3.71%)	⬇️
catalog/cataloger_create_entry.go	100.00% <0.00%> (+5.26%)	⬆️

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