Add better header manipulation support (e.g., for GETs)

[natoverse]

Request headers can only be natively manipulated for methods with an entity payload (e.g., PUT/POST). The 'request' plugin type should always run, even for GET & DELETE so that headers can be set.

In the meantime, a workaround could be to extend one of the downstream classes (Request, or a DataProvider of choice) and inject the 'headers' field into the params.