Island: Add /api/register-agent endpoint

[cakekoa]

What does this PR do?

Fixes part of #3076.

Adds mock endpoint for getting token (requires OTP).

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

[mssalvatore]

also for this: #3083 (comment)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.04 🎉

Comparison is base (c254126) 67.58% compared to head (69eff25) 67.62%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3084      +/-   ##
===========================================
+ Coverage    67.58%   67.62%   +0.04%     
===========================================
  Files          444      445       +1     
  Lines        12730    12751      +21     
===========================================
+ Hits          8603     8623      +20     
- Misses        4127     4128       +1     

see 4 files with indirect coverage changes

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[mssalvatore]

After much discussion, we settled on GET /api/agent-otp and POST /api/agent-otp-login.