Support OPA as an Authorizer trait implementation

[autodidaddict]

Provide configurable support for an implementation of the Authorizer trait to defer authorization decisions to an OPA policy. Part of this configuration will need to be how to interact with OPA, e.g. the URL/credentials for the OPA URL.

We can do some comparisons to see if it's worth hiding behind a feature flag since the OPA interaction would basically be limited to making HTTP client calls.