R4R: client/utils/ reorganization

[alessio]

Move REST-related types and functions out of client/utils into a new client/rest package.

• 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

---

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 #3487 into develop will decrease coverage by 6.42%.
The diff coverage is 80%.

@@             Coverage Diff             @@
##           develop    #3487      +/-   ##
===========================================
- Coverage    59.44%   53.02%   -6.43%     
===========================================
  Files          130      137       +7     
  Lines         9804    10733     +929     
===========================================
- Hits          5828     5691     -137     
- Misses        3606     4692    +1086     
+ Partials       370      350      -20

[cwgoes]

ACK - sure, this seems a bit cleaner

[jleni]

If this is ready to go, please merge and I will rebase my PR that has some conflicting/similar changes.