Handle OPA/Gatekeeper policies

[flavio]

This card is part of kubewarden/policy-evaluator#14.

Policy server should be able to load and evaluate Wasm modules that have been originated by opa build and have been then annotated via kwctl (see kubewarden/kwctl#55).

Admission criteria

• policy server will load Rego-based policies that have been annotated by kwctl
• A Rego-based policy that has NOT been annotated by kwctl will not be loaded, and will cause the server to exit with an error
• policy server will prepare the right values expected by OPA and Gatekeeper policies

[flavio]

To be more specific:

• policy-server requires all the wasm policies to be annotated via kwctl annotate. This is the current behavior and we don't intend to change it
• If the kubewarden metadata includes information about the runtime to use: we will use this information to pick the right runtime (kubewarden/opa/gatekeeper)
• If the kubewarden metadata does NOT include information about the runtime to use (such as all the policies that currently exist), then we will assume the policy is a kubewarden one, and use that runtime