text: Title: Add convenience methods on `ParameterSet`
  Body: 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](https://github.com/schemathesis/schemathesis/blob/master/src/schemathesis/parameters.py#L55) 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:
  
  ```python
  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`