* adding LocalRootSpan class
this is based on Java's implementation, https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/v2.3.0/instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/instrumenter/LocalRootSpan.java
and adds the ability to identify and locate the "local root span" in a trace. The local root span is the top-level active span which has either
an invalid or remote parent.
It's tracked automatically as part of making a span active, either via `Span::activate()` or `Span::storeInContext()`, and can be retrieved in
a couple of ways, but most easily via `LocalRootSpan::current()`.
* remove redundant local root span check
* move context key to api
* internal
* adding example
* mark LocalRootSpan as experimental
* adding an example of local root span usage
* style, fix broken build
Brett McBride