RBAC package and model to craft policies

[shashankram]

Please describe the Improvement and/or Feature Request
#1964 implemented server side RBAC policies as a part of LDS. This needs to be extended out as a separate package to be able to build in XDS RBAC policy, preferably using a data model. This will also be used by RDS to build RBAC policies per route.

Scope (please mark with X where applicable)

• New Functionality [X]
• Install [ ]
• SMI Traffic Access Policy [ ]
• SMI Traffic Specs Policy [ ]
• SMI Traffic Split Policy [ ]
• Permissive Traffic Policy [ ]
• Ingress [ ]
• Egress [ ]
• Envoy Control Plane [X]
• CLI Tool [ ]
• Metrics [ ]
• Certificate Management [ ]
• Sidecar Injection [ ]
• Logging [ ]
• Debugging [ ]
• Tests [ ]
• CI System [ ]
• Project Release [ ]