feat(contrib/cloudevents): Provide helpers for tracing for cloudevents

[alfredosa]

What does this PR do?

CloudEvents is a very popular framework now to work with events, in an abstracted way. dd-trace-go does not have a native way to propagate context through CloudEvents.

I propose adding contrib/cloudevents/sdk-go.v2 which would provide:

• Context Injection: functions to work and properly inject span context (mapping to W3C/TraceParent).
• Context Extraction: functions to extract said data
• Publisher Wrapper: aid with a wrapper and inject the context (as cloudevents extensions)
• Handler Wrapper: A function for cloudevents.Client or handler functions to automatically start spans on incoming events.

Motivation

I think this would benefit every other adopter of cloud events! I opened #4137 to discuss but also mentioned I can provide the code, In case it's useful in advanced to expedite the discussion and get feedback.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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