Support publishing and consuming policy packs using any runtime

[lukehoban]

Fixes #5089.

[lukehoban]

Tested this out using the OPA runtime.

Publishing:

$ /opt/pulumi/bin/pulumi policy publish    
Obtaining policy metadata from policy plugin
Compressing policy pack
Uploading policy pack to Pulumi service
Publishing "kubernetes" to "lukehoban"
Published as version 1

Permalink: https://app.pulumi.com/lukehoban/policypacks/kubernetes/1

Consuming:

$ /opt/pulumi/bin/pulumi up
Previewing update (dev2):
     Type                           Name                    Plan       Info
 +   pulumi:pulumi:Stack            simple-kubernetes-dev2  create     1 error
 +   ├─ kubernetes:core:Pod         myapp                   create     
 +   └─ kubernetes:apps:Deployment  nginx                   create     
 
Diagnostics:
  pulumi:pulumi:Stack (simple-kubernetes-dev2):
    error: preview failed
 
Policy Violations:
    [advisory]  kubernetes v0.0.1  warn (nginx: kubernetes:apps/v1:Deployment)
    nginx-5kjh7i6n must include Kubernetes recommended labels: https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/#labels 
    
    [mandatory]  kubernetes v0.0.1  deny (myapp: kubernetes:core/v1:Pod)
    image 'nginx' comes from untrusted registry
    
    [mandatory]  kubernetes v0.0.1  deny (myapp: kubernetes:core/v1:Pod)
    image 'mysql' comes from untrusted registry

[justinvp]

Nice