add support for client authentication; fix e2e client tests

[wrieg123]

Adds support for username & password authentication of client. Fixes e2e tests.

Tests Added

test_supervisor_client -> tests unauthenticated client e2e
test_permissioned_supervisor_client_rejected -> tests that bad login information raises and exception in the call
test_permissioned_supervisor_client -> tests authenticated client e2e

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
airflow_supervisor/client/xmlrpc.py	67.78% <100.00%> (+0.21%)	⬆️
airflow_supervisor/config/airflow.py	76.31% <ø> (ø)
airflow_supervisor/tests/client/test_client_e2e.py	100.00% <100.00%> (ø)
airflow_supervisor/tests/conftest.py	100.00% <100.00%> (ø)