Open policy agent support for Teleport

Embed it as a library to support RBAC definitiions, people can create OPA roles in our RBAC:

role: v3
name: opa-policy
   policy | 
      package application.authz

      # Only owner can update the pet's information
      # Ownership information is provided as part of OPA's input
      default allow = false
      allow {
          input.method == "PUT"
           some petid
           input.path = ["pets", petid]
           input.user == input.owner

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