Support traffic access control

[localvar]

In #58 , we wish to support service mesh interface. However, there are gaps between the concepts used by Easemesh and the concepts in SMI which are difficult to span.

But, it is possible to implement some SMI features in Easemesh by an alternative solution, and this issue is created to track the design and implementation of a feature to Traffic Access Control.

Differences between SMI Traffic Access Control and Easemesh Traffic Access Control are:

• For SMI Traffic Specs, Easemesh only support HTTPRouteGroup.
• In SMI, traffic access control is enforced on the server-side (traffic target), Easemesh will enforce it on the client-side (traffic source).
• In SMI, access is controlled based on K8s service accounts; in Easemesh, access is controlled based on Easemesh services.
• Easemesh will not support the namespace of a traffic source or target in SMI

Below is an example spec of TrafficTarget in Easemesh:

---
kind: HTTPRouteGroup
metadata:
  name: the-routes
spec:
  matches:
  - name: metrics
    pathRegex: "/metrics"
    methods:
    - GET
  - name: everything
    pathRegex: ".*"
    methods: ["*"]

---
kind: TrafficTarget
metadata:
  name: path-specific
spec:
  destination:
    kind: Service
    name: order
  rules:
  - kind: HTTPRouteGroup
    name: the-routes
    matches:
    - metrics
  sources:
  - kind: Service
    name: monitor

[zhao-kun]

Closed due to https://github.com/megaease/easegress/pull/308