Consider supporting Open Policy Agent Rego as the policy language for warrant policies

[kkajla12]

Is your feature request related to a problem? Please describe. Now that #140 is merged and warrants support attaching policies to them, it might make sense to support additional policy languages like OPA's Rego (due to wider adoption/familiarity).

Describe the solution you'd like Warrants should support policies written in OPA's Rego policy language.

Describe alternatives you've considered N/A

Additional context It seems like using OPA purely for policy definition (in Rego) and evaluation is possible in Go services via their Go API: https://www.openpolicyagent.org/docs/latest/integration/#integrating-with-the-go-api