Investigate the feasibility of non-explicit-list access controls; i.e. instead of admins explicitly listing usernames to allow/deny access to Cody, derive this information from IdP, Okta, RBAC, cost host user groups, or other sources of user group management.

Note: the goal of this issue is to figure out what we could implement in a timely manor, which of these would overlap well with customers, and to have more in-depth conversations about this feature with product (Ryan Phillips) & customers (ideally we find a single customer to build this for first) to identify which reasonable solutions we can implement.

See https://github.com/sourcegraph/sourcegraph/issues/58628 for the tracking issue for implementing this behavior, if we decide the juice is worth the squeeze.