Add runtime support for Knative sinkbinding

[nicolaferraro]

Sinkbinding allows to use an existing Camel K Integration as as a standard Knative source (it's listed among sources and compatible with the kn tools), without having to use the CamelSource object.

To make the user experience good, there are some things to hammer here and there, but we can already enable them if we do some changes in the runtime.

E.g. we can run an integration named routes, then provide the following sinkbinding:

apiVersion: sources.knative.dev/v1alpha1
kind: SinkBinding
metadata:
  name: bind-camel-routes
spec:
  subject:
    apiVersion: apps/v1
    kind: Deployment
    name: routes
  sink:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: event-display
  ceOverrides:
    extensions:
      sink: bound

It targets the underlying deployment created by the integration, but we can work to have also the Camel K Integration usable as subject in the future.

The contract of the SinkBinding spec says that information about the target endpoint will be injected in the following envs of the running pod:

• K_SINK: target URL for sending cloud events (note that we already use KNATIVE_SINK to indicate the logical name of the sink in the loader, we can rename that to avoid confusion)
• K_CE_OVERRIDES: header information to override

We should configure the knative component to use those properties if available, before looking in the general config (or if there's no match there).