Allow users to configure a brave based SDK with auto-instrumentation

[bogdandrutu]

Once open-telemetry/opentelemetry-java#1735 is done, auto-instrumentation should support configuring the brave "sdk" implementation as one of the possible implementation of our API. Users will be able to choose between our SDK or brave base SDK.

[trask]

Closing, can revisit later if anyone has renewed interest.