Add support for header-based filtering on TrafficTargets

[jspdown]

What does this PR do?

This PR adds support for header-based filtering on TrafficTargets. As described in the specification, an HTTPRouteGroup can define a set of headers that must match to pass through.

https://github.com/servicemeshinterface/smi-spec/blob/master/apis/traffic-specs/v1alpha3/traffic-specs.md#http-header-filters

kind: HTTPRouteGroup
metadata:
  name: the-routes
  namespace: default
spec:
  matches:
  - name: android-insiders
    headers:
    - user-agent: ".*Android.*"
    - cookie: "^(.*?;)?(type=insider)(;.*)?$"

How to test it

• make test
• make test-integration
• Install maesh and play with this feature. Instruction on how to use this new filter can be found in the specification