Change client.go's `subscription` to better reflect the Trigger Filter attributes

[matzew]

Currently the client.go file contains a Subscription:

type Subscription struct {
	Source string `json:"source" yaml:"source"`
	Type   string `json:"type" yaml:"type"`
	Broker string `json:"broker" yaml:"broker"`
}

This struct is wrong, since it only represents the source and/or type attributes on the Trigger.Filter, however that can be anything, no requirement on source or type as a criteria.

[matzew]

See also https://github.com/knative/eventing/issues/7265

[matzew]

It would be nice, if for func describe things would render like:

Subscriptions (Broker and Attributes):
  my-broker, map[source:/something type:org.apache.camel.event.aws.s3.getObject extension:someOther]

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