Switch the openapi-codegen to our docker image

[svix-mman]

Using the docker image from svix/openapi-codegen#82
This results in the identical code.

I replaced the regen_openapi.sh with a multi-threaded python script

[svix-mman]

CI is broken because it's trying to pull an image we have not yet published
CI is slow because I added a docker build command that take 10-15 minutes
Once I merge svix/openapi-codegen#82 this will simply be a docker pull

[svix-jplatte]

This PR is marked as ready for review but blocked on several other PRs + contains a replacement of the bash script with no explanation as to why (and doesn't delete the old script either). I'm guessing it is not in fact ready for review?

[svix-mman]

This PR is marked as ready for review but blocked on several other PRs + contains a replacement of the bash script with no explanation as to why (and doesn't delete the old script either). I'm guessing it is not in fact ready for review?

yeah :)
let me mark it as draft