Alternative integration/API testing tool to Postman

I like Postman and how easy it is to create tests in there. But there are things that are quite hard to make work in there. One of those is the use of source control. Looks like the collections are saved somewhere and the files are placed somewhere. I have to do export/import. We have an existing code repository and I just want to create a sub-directory in that repository where all of the test-related files will be put in there.

Quite a lot of our endpoints have massive number of fields in JSON format and results will greatly vary depends on the the provided data. I need something that I can test multiple scenarios and compare specific fields to expected result. How I do it on Postman is duplicate an existing request, change its body and test based on the new scenario.

Is there a tool that is better than Postman in these features? I prefer an open source solution because aside from being free, I can peek at the code or even submit fixes/features if possible. Thanks!