Add limited Okta connection pooling

[barborico]

Motivation

This PR introduces limited connection pooling for requests from Access to Okta. The intent is to avoid creating excessive connections, e.g. in case of Okta API degradation resulting in timeouts. Although the aiohttp docs for ClientSession state that "it is suggested you use a single session for the lifetime of your application to benefit from connection pooling," this isn't feasible due to the synchronous nature of the Access Flask app. Instead, we introduce a session with each async event loop to call the Okta API. This should at least consolidate retries and paginated requests to a single connection.

Reviewing

Hide whitespace, since most of the changed lines are simply indented.