Add OpenAPI code generation example

[stanislav-zeman]

The REST API lecture currently gives an example of generating OpenAPI from Go code annotations. The lecture currently only provides a list of libraries that can be used for generating code, i.e. the opposite operation. It would be nice to have a little showcase of the generated code and how it is done.

[Matovidlo]

# Install if not already installed                                                                                                            
go install github.com/deepmap/oapi-codegen/cmd/oapi-codegen@latest                                                           
  
# Generate Go client
oapi-codegen -package manage -generate types,client open-api.yaml > your_client.go

Something like this? https://github.com/oapi-codegen/oapi-codegen

[stanislav-zeman]

Yep. I believe the oapi-codegen is actually mentioned. However, there is no code showcase so the students could get an idea of what the output looks like.