External evaluation design

[sstults]

Description

This PR adds design documentation for #120.

Issues Resolved

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[fen-qin]

@sstults @epugh
Thanks for putting so many details in the design documentations.

Would you like to address the following questions ?

• is there any restriction/validation for external experiment results ?
  • limits on the total payload size
  • the number of evaluation results
  • the size of individual fields.

Would you like to add proper request throttling mechanisms to prevent abuse of the import API ?
Would you like to add proper validation/sanitization to make sure no craft experiment results containing code or scripts that might be executed when viewed or processed ?

I will copy these to implementation PR as well. thanks