Add cognito refresh token as an accepted auth method

[cranti]

I added some unit tests, and I have been testing the refresh logic locally. I'd also love for a reviewer to install from this branch & test it!

With a refresh token & client ID, here's an example of the type of testing I've been doing...

from pprint import pprint

from runeq import initialize
from runeq.resources.user import get_current_user
from runeq.resources.stream_metadata import get_patient_stream_metadata
from runeq.resources.org import set_active_org

initialize(
    cognito_refresh_token='TODO', 
    cognito_client_id='TODO'
)

# make some GraphQL requests
print(get_current_user())
set_active_org('TODO_ORG_ID')

# fetch a little bit of data, just to use the Stream API
all_sm = get_patient_stream_metadata(patient_id='TODO_PATIENT_ID')
for sm in all_sm:
    for data in sm.iter_stream_data(format='json', limit=5):
        pprint(data)
        print('--')
    break

[vijaveinbergs]

lgtm! tested locally with a refresh token and was able to fetch some data