Add missing implementations for operation functions in examples

[berislavlopac]

The src/examples/server/pets.py file contains stubs for all the functions corresponding to the the operation IDs as defined in the petstore.yaml specification. However, only find_pets_by_status has any implementation, while all others just have pass, meaning that they would in most cases cause a validation error when sending a response.

A good first issue would be to implement those remaining functions in line with the responses specified in the petstore.yaml, with correct response codes and contents (if any).