Introduce OpenTelemetry observability service

[joaopgrassi]

Resolves #554, #693

This PR

• Adds a new implementation of the ObservabilityService interface for OpenTelemetry. The new observability service is similar to the existing one for OpenCensus.
• Exposes inject and extract functions to read and write W3C Trace Context data to/from events respectively.
• Exposes helper methods to create (and extend) CloudEvents HTTP clients enabled with OpenTelemetry
• A sample project demonstrating how to use the "observed" client with OpenTelemetry (with spans that can be viewed with Jaeger)
• Tests (in a separated module to not impose the OTel default SDK as a transitive dependency for users)

Note: The observability service is only producing spans at the moment. Metrics support can be added once the SDK is more stable.

[n3wscott]

AWESOME!! I am reviewing now, give me a day or two, thanks so much!

[joaopgrassi]

@n3wscott there might be some changes related to the attributes that are added to the spans created by the ObservabilityService. It's currently being discussed in my PR on the OTel spec. In case you want to see it: open-telemetry/opentelemetry-specification#1978

Not sure which is going to be reviewed/merged first, but if this is merged first, then I can change it later to adhere with the OTel spec :).

[n3wscott]

amazing work, with tests, samples and READMEs and everything!! I can't wait to use this instead of the open census integration.