Add semantic conventions for CloudEvents

[joaopgrassi]

The CloudEvents Go SDK exposes an interface that is used by the CloudEvents client to add observability to the actions performed by it. For example: The client invokes the observability service when sending an event.

Currently, the SDK offers an implementation of the interface that works with OpenCensus. In there, a couple of CloudEvents specific attributes are added to the spans.

I've opened a PR to add a new implementation for OpenTelemetry and I thought that it would be good if these attributes could be part of the OpenTelemetry semantic conventions.

[pyohannes]

As discussed in today's messaging WG, please add CloudEvents attributes as separate semantic convention. This way it can be utilized as an add-on for both messaging and HTTP scenarios.