Support target authentication

The `send` command currently assumes no authentication in the endpoint. `kn event send` is useful outside of direct Knative cluster access, e.g. to send Cloud Events to arbitrary webhooks or endpoints (services) in Kubernetes/Knative which enforce authentication, e.g. basic_auth.

What do you think about adding support for authentication schemes?

Start with the `send` command, which currently assumes unauthenticated endpoints, and trace how it constructs requests. Define the authentication schemes and configuration needed for authenticated webhooks or Kubernetes/Knative services, then verify that `kn event send` can deliver Cloud Events to them.

Written by the indexing model from the issue text.