[server] Automatically add OPTIONS method alongside any other method

[berislavlopac]

Is your feature request related to a problem? Please describe.
CORS requires an OPTIONS request to be sent before an actual request to an endpoint.

Describe the solution you'd like
It might be useful to have pyotr server automatically create an additional OPTION endpoint for each operation, which would return the operation spec.

Additional context
It might be an app-wide setting, on by default