Support Trigger Filters attribute

[matzew]

It looks like the new (since 1.15) GA feature of Trigger.filters (plural) is not supported on latest kn client

/kn-linux-amd64 version 
Version:      
Build Date:   
Git Revision: 
Supported APIs:
* Serving
  - serving.knative.dev/v1 (knative-serving v0.43.0)
* Eventing
  - sources.knative.dev/v1 (knative-eventing v0.43.0)
  - eventing.knative.dev/v1 (knative-eventing v0.43.0)
➜  Downloads ./kn-linux-amd64 trigger create --help
Create a trigger

Usage:
  kn-linux-amd64 trigger create NAME --sink SINK [options]

Examples:

  # Create a trigger 'mytrigger' to declare a subscription to events from default broker. The subscriber is service 'mysvc'
  kn trigger create mytrigger --broker default --sink ksvc:mysvc

  # Create a trigger to filter events with attribute 'type=dev.knative.foo'
  kn trigger create mytrigger --broker default --filter type=dev.knative.foo --sink ksvc:mysvc


Options:
      --broker string      Name of the Broker which the trigger associates with. (default "default")
      --filter strings     Key-value pair for exact CloudEvent attribute matching against incoming events, e.g type=dev.knative.foo
  -n, --namespace string   Specify the namespace to operate in.
  -s, --sink string        Addressable sink for events. You can specify a broker, channel, Knative service, Kubernetes service or URI. Examples: '--sink broker:nest' for a broker 'nest', '--sink channel:pipe' for a channel 'pipe', '--sink ksvc:mysvc:mynamespace' for a Knative service 'mysvc' in another namespace 'mynamespace', '--sink
                           https://event.receiver.uri' for an HTTP URI, '--sink ksvc:receiver' or simply '--sink receiver' for a Knative service 'receiver' in the current namespace, '--sink svc:receiver:mynamespace' for a Kubernetes service 'receiver' in the 'mynamespace' namespace, '--sink special.eventing.dev/v1alpha1/channels:pipe'
                           for GroupVersionResource of v1alpha1 'pipe'. If a prefix is not provided, it is considered as a Knative service in the current namespace.

Use "kn-linux-amd64 options" for a list of global command-line options (applies to all commands).

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