Pass through extra headers on every request.

[lsb]

No description provided.

[lsb]

Some manual testing:

>>> Client(host="scribd.external.verta.ai", email="leeb@scribd.com", dev_key="12341234-1234-1234-1234-123412341234")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/leeb/modeldb/client/verta/verta/client.py", line 199, in __init__
    _utils.raise_for_http_error(response)
  File "/Users/leeb/modeldb/client/verta/verta/_internal_utils/_utils.py", line 681, in raise_for_http_error
    six.raise_from(requests.HTTPError(message, response=response), None)
  File "<string>", line 3, in raise_from
requests.exceptions.HTTPError: 403 Client Error: Forbidden for url: https://scribd.external.verta.ai/api/v1/modeldb/project/verifyConnection at 2021-07-23 20:03:21.724000 UTC
>>> Client(host="scribd.external.verta.ai", email="leeb@scribd.com", dev_key="12341234-1234-1234-1234-123412341234", extra_auth_headers={"12341234123412341234": "12341234-1234-1234-1234-123412341234"})
connection successfully established
<verta.client.Client object at 0x10b373670>

[convoliution]

Nice, thanks again @lsb

I'll have this merged come Monday and start running it through our tests.