pkg/*: add api to list traffic targets composed of associated routes

[shashankram]

Description:
Adds an API to retrieve a list of traffic target objects along
with their associated routes (limited to TCP). The traffic target
objects will be used to directly map to RBAC policies configured
on the upstream proxy.

This API will be leveraged to implement RBAC policies for TCP traffic.
For ex., the following TrafficTarget policy and TCPRoute will generate
an RBAC policy as seen below.

kind: TCPRoute
metadata:
  name: tcp-route
  namespace: default
spec:
  matches:
    ports:
    - 8080
---
kind: TrafficTarget
metadata:
  name: test
  namespace: default
spec:
  destination:
    kind: ServiceAccount
    name: sa-1
    namespace: default
  rules:
  - kind: TCPRoute
    name: tcp-route
  sources:
  - kind: ServiceAccount
    name: sa-2
    namespace: default
---

rbac_policy on proxy with identity default/sa-1

{
	{
		name: "test"
		principals: {"default/sa-2"},
		permissions: {destination_port: 8080},
	},
}

Part of #1521

Affected area:

• New Functionality [X]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?
  No