This PR adds mediocre support for setting request ids to support request debugging.

Methods on censys.search.{CensysHosts|CensysCerts} don't support configuring the underlying request object on each call. Given that **kwargs is used to glob additional search params on most methods, I didn't see a clean and consistent way to support setting one-off request options through the query methods themselves.

Support for setting the x-request-id header is implemented by adding a property to the CensysAPIBase class, where updates will mutate the request objects headers, so that the next request will include the assigned request-id value.

This leads to an awkward, though functional, usage pattern:

from censys.search import CensysHosts

h = CensysHosts(request_id="test-id-0")
host = h.view("8.8.8.8")                    # request with  x-request-id: test-id-0

h.request_id = "test-id-1"
host = h.view("8.8.8.8")                    # request with  x-request-id: test-id-1

host = h.view("8.8.8.8")                    # request with  x-request-id: test-id-1

h.request_id = "test-id-2"
host = h.view("8.8.8.8")                    # request with  x-request-id: test-id-2

h.request_id = None
host = h.view("8.8.8.8")                    # request without x-request-id

h.request_id = "test-id-3"
host = h.view("8.8.8.8")                    # request with  x-request-id: test-id-3

This ux is definitely not ideal, but it makes the feature available quickly with minimal changes to the package interfaces. I figure this is tolerable for now since it's only meant for debugging.