Is it possible to create a plugin to embed in the TestRail cloud?
What are you trying to accomplish? There are plug ins already available to integrate TR and various defect trackers like Jenkins
I have a python script that uses the TestRail API (v188.8.131.52) to migrate the results between Test Runs.
I am trying to make it into a plugin in TestRail Cloud.
afaik it is not possible to add any plugin in the cloud instance. You can have an external tool using the api and maybe you can integrate that via UI script (http://docs.gurock.com/testrail-custom/uiscripts-introduction)