Contrib API proposal

[darccio]

What does this PR do?

Proposes a contrib API design for v2 contribs. Think of it as a support package to create contribs without depending on internal packages.

Motivation

Unblock the release of v2 as contribs are going to be released as isolated nested modules.

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.
• 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.

Unsure? Have a question? Request a review!