This PR adds an RBAC package for managing using permissions:
- The top-level `authz.Authorize` function is the main user-facing entrypoint to the package.
- Actual permission evaluation is handled in `policy.rego`.
- Unit tests for `authz.Authorize` are in `authz_test.go`
- Documentation for the package is in `README.md`.
Co-authored-by: Cian Johnston <cian@coder.com>
