Check openapi correspondence

[ch1bo]

Why

While we use servant to describe the API types in the hydra-explorer, the actual api/*.yaml openapi schemas are the interface to be used by observers and clients.

We want to ensure that the implementation (through servant-server) is matching what is claimed in the openapi schema through some automated testing (and stays that way).

What

• Add a test suite that fails if we change the ObservationApi Haskell type (or its constituents), but not the openapi schema in observer-api.yaml#paths/" /observations/{network}/{version}"
• Add a test suite that fails if we change the ClientApi Haskell type (or its constituents), but not the openapi schema in client-api.yaml#paths/\/ticks or client-api.yaml#paths/\/heads

[ch1bo]

This looks like it could be what we want: https://schemathesis.readthedocs.io/en/stable/getting-started.html