Add root span/scope tracking

[SammyK]

Description

This PR adds a way to track the root span so that it can be modified later in the script execution. This is important for integrations that don't have a way of hooking into the request/routing/etc early on in the script execution when the tracer is inited.

It works just like Tracer::startActiveSpan(), but it saves the Scope instance on the tracer to be accessed later by Tracer::getRootScope().

$tracer->startRootSpan('my_root_span');
// ...do lots of stuff...
$scope = $tracer->getRootScope();
// ..add tags and whatnot

Readiness checklist

• Changelog entry added, if necessary
• Tests added for this feature/bug

[labbati]

I really like the idea @SammyK , just left a few comments. 👍

[labbati]

Awesome I l ike it! Thanks @SammyK 🔝