Custom API endpoint for plugins

[touhidurabir]

Description:
Details and related discussions at #11991

What application are you using?
OJS, OMP or OPS version 3.5.0 and main

PRs

Stable-3.5.0

pkp-lib --> #12051
ojs --> pkp/ojs#5185 [TEST ONLY]

Example Usage

touhidurabir/apiExample#1

[touhidurabir]

@ewhanson can you please review the PRs at #12050 (comment) . we can port to main after we finalise the approach for the 3.5.0 branch .