Ability to select multiple specific endpoints

[Stranger6667]

Currently, we have two filters - endpoint and method, but what if we want to test only GET /api/users and POST /api/orders? It could be somehow done with method=["GET", "POST"] and endpoint=["/api/users", "/api/orders"] but it will also select GET /api/orders which we might not want.

It would be nice if the endpoint option can accept a list of tuples, where each tuple is a pair of method and path, then it might be possible to select any specific endpoint for Open API.

Implementation notes

• Update the type for BaseSchema.endpoint to include such tuples.
• It might be easier to create a separate "filter" function in schemathesis.specs.openapi.filters that will accept the full path, method, and endpoints in this format
• This function might be called in this condition and internally will be a no-op if endpoints are passed in the regular format.
• At the same time, the current should_skip_endpoint function might be a no-op for this new format. The reason for this complication is to avoid extra work of resolving methods and dispatching the "before_process_path" hook.