Add custom implementation of operation

[maxlapshin]

Summary

Want to add some VERY primitive dynamic behaviour of API (show download progress).

Are there any ways to get into prism response generation mechanism and put there some hooks?

[chohmann]

@maxlapshin we don't feel that this functionality fits the spirit of what Prism should be. Relates to #1838.

[maxlapshin]

No problems, I just cannot use prism and I do not use it, because it is practically not useable =)

I took openapi sampler and it helped me to write server that can simulate real server.