Add Open Policy Agent integration

[JaneJeon]

I would like to move off of CASL due to the lack of UI support (and the fact that it has a bus factor of 1), and OPA seems to be basically the only “real” actively developed one with some sort of multi-language support (which rules out drools), but as of now, the WASM support (which we need to evaluate policies) is experimental.

https://www.openpolicyagent.org/docs/latest/integration/#comparison