Add URI validation for EventType Source and Schema fields

[Kunal1522]

The EventTypeSpec validation in pkg/apis/eventing/v1beta1/eventtype_validation.go (and v1beta2, v1beta3) has TODO comments indicating that Source and Schema fields should be validated as proper URIs:
https://github.com/knative/eventing/blob/main/pkg/apis/eventing/v1beta1/eventtype_validation.go#L36-L37

// TODO validate Source is a valid URI.
// TODO validate Schema is a valid URI.

[Kunal1522]

/assign