Create python-flask openapi generator template so that controllers aren't overwritten

[thomasyu888]

Currently, there is quite a bit of work in having to undo the changes that the openapi-generator does, but if we created our own templates to import from a core module within the package that is unchanged, then it wouldn't be an issue.

Essentially this is the command I run to generate a service:

openapi-generator generate -i openapi.yaml -g python-flask -o server -t .codegen/server/

To get to this point, you will want to:

1. Create your own templates

   openapi-generator author template -g python-flask -o .codegen/server

2. Edit the templates in these two places:
   • add import
   • update return
3. Create core module by copying everything in controllers in your packages into core. example
4. Run the generation command above and get something like this: example. Notice how everything in controllers now just pulls from core.controllers.queue_controller?

[tschaffter]

We should create a GH repository where can be develop these template and either publish our custom openapi-generator to npmjs and/or Docker Hub (see openapi-generator Dockerfile).

[tschaffter]

See https://github.com/nlpsandbox/date-annotator-example/pull/96#issuecomment-802553660