Add AsyncAuth0 to share a session among many services

[adamjmcgrath]

Changes

Add feature to share a single session across multiple services in Auth0

async with Auth0('domain', 'mgmt_api_token') as auth0:
    user = await auth0.users.get_async(user_id)
    connection = await auth0.connections.get_async(connection_id)

# or

auth0 = Auth0('domain', 'mgmt_api_token');
session = SingletonAuth0AiohttpClientSession.get_aiohttp_client_session()
auth0.set_session(session)
user = await auth0.users.get_async(user_id)
connection = await auth0.connections.get_async(connection_id)
await session.close()

References

fixes #317

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[adamjmcgrath]

Build is failing because keybase.io is down

[Widcket]

LGTM, just fixed a Latin abbreviation in the API docs.