Prevent the need for repetition in sample definitions

[txels]

Allow one to skip repeating the request URL in all samples, which is the case when the URL of the endpoint under tests is fixed (contains no variable parts).

E.g. this is an example from a project I'm working on:

{
    "description": "Retrieve profile of logged in user",
    "url": "/accounts/me",
    "method": "GET",
    "examples": {
        "OK": {
            "response": {
                "status": 200,
                "body": {
                    "id": 1,
                    "username": "user-0",
                    "first_name": "",
                    "last_name": "",
                    "email": "user-0@example.com"
                }
            }
        },
        "unauthenticated": {
            "description": "I am not logged in",
            "response": {
                "status": 403,
                "body": {
                    "detail": "Authentication credentials were not provided."
                }
            }
        }
    }
}

Before this PR, I had to write:

{
    "description": "Retrieve profile of logged in user",
    "url": "/accounts/me",
    "method": "GET",
    "examples": {
        "OK": {
            "request": {
                "url": "/accounts/me"
            },
            "response": {
                "status": 200,
                "body": {
                    "id": 1,
                    "username": "user-0",
                    "first_name": "",
                    "last_name": "",
                    "email": "user-0@example.com"
                }
            }
        },
        "unauthenticated": {
            "description": "I am not logged in",
            "request": {
                "url": "/accounts/me"
            },
            "response": {
                "status": 403,
                "body": {
                    "detail": "Authentication credentials were not provided."
                }
            }
        }
    }
}

...which includes some tedious repetition.

[txels]

@nwhite89 this change somehow modifies the semantics of an ABE file, as it defines the meaning when a request section is missing.

This made me realise that in ABE-spec, we need to document better what all fields mean, what is optional and mandatory, etc.

Also, this PR may imply a change in the format version and as such we need to update the JS packages that use the format.

Let me know what you think. Maybe we want to start with a PR to the spec?