Decouple instrumentation from default SDK

[MrAlias]

Instrumentation needs to be lightweight when used. It should not have any large dependencies. However, there are multiple instrumentation packages that import the go.opentelemetry.io/otel/sdk package. Most common this is in examples or tests.

This dependency needs to be removed. It will likely depend on the resolution of open-telemetry/opentelemetry-go#872.

[matej-g]

It seems now also with tracing utilities being available (open-telemetry/opentelemetry-go#1111), nothing should be blocking this from go.opentelemetry.io side, right? If so I'd be happy to take it.