R4R: Allow Simulation and Gen Only in REST Client

[alexanderbez]

closes: #3423

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• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3467 into develop will increase coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #3467      +/-   ##
===========================================
+ Coverage    59.51%   59.52%   +<.01%     
===========================================
  Files          135      135              
  Lines         9981     9973       -8     
===========================================
- Hits          5940     5936       -4     
+ Misses        3669     3665       -4     
  Partials       372      372

[alessio]

Just a small thing (promoting client/utils/errors.go to client). Nihil obstat from me otherwise

[alexanderbez]

Yup -- I'll update @alessio