Add convenience methods on ParameterSet

[Stranger6667]

It could be useful in hooks, e.g. to check whether APIOperation contains some header (we leave case-insensitive for a while).

1. Add a new contains method to the ParameterSet class, the method should accept name of type str
2. The implementation should reuse the get method and check whether its result is None
3. Inside test/specs/openapi/parameters/test_simple_payloads.py add a new function test_parameter_set_get
4. This test function could have a setup like this:

import schemathesis

# ... other tests omitted for brevity

def test_parameter_set_get(make_openapi_3_schema):
    header = {"in": "header", "name": "id", "required": True, "schema": {}}
    raw_schema = make_openapi_3_schema(parameters=[header])
    schema = schemathesis.from_dict(raw_schema)

5. Use the new method from step 1 on schema["/users"]["POST"].headers inside this test and verify that it contains the header with the name id and does not contain the header with the name unknown

[EkaterinaZaritskaia]

Hi, I'll try to solve it