[FEATURE] Support the discriminator keyword properly

[Stranger6667]

Is your feature request related to a problem? Please describe.
Schemathesis does not use discriminator at all, but it has certain behavior in OAS 3.0. Specifically:

If the discriminator value does not match an implicit or explicit mapping,
no schema can be determined and validation SHOULD fail

and

In this case, a discriminator MAY act as a "hint" to shortcut validation and selection of 
the matching schema which may be a costly operation, depending on the complexity of the schema

If these aspects are applied, then some responses previously considered invalid will be valid. Though, as the latter case uses "MAY", the current behavior of Schemathesis is OK in this regard (the former case, should be implemented anyway)

Describe the solution you'd like
Schemathesis uses discriminator to shortcut to the right schema instead of running the regular JSON Schema validation. It would be hard to do it dynamically, as schemas are recursive and I don't want to hook into the validation process. Instead, I think it could be emulated with if & anyOf:

"anyOf": [
    {
        "if": {
            "properties": { "petType": { "const": "Cat" } }
        },
        "then": {
            "$ref": "#/components/schemas/Cat"
        },
    },
    {
        "if": {
            "properties": { "petType": { "const": "Dog" } }
        },
        "then": {
            "$ref": "#/components/schemas/Dog"
        },
    }
]

Or just with if / then / else, which may produce better error messages. Not sure yet

Additional context
I missed a couple of aspects of the Open API spec in this regard