Allow customization of parent-override behaviour for inferred-spans

[JonasKunz]

Description:

The inferred spans extension sometimes has to insert inferredspans inbetween the parent-child relationship of normal spans for correctness.

This is however not directly possible, that is why we represent this as a span-link on the inferred spans. This PR adds a configuration hook to fully customize this behaviour with a custom handler.

The main reason for adding this is because in our elastic vendor distro of the opentelemetry agent we are still shipping the pre-contribution version of inferred spans. We currently still have to do this, because there these special span links are marked with an elastic.is_child instead of just is_child attribute and older versions of our backend rely on this elastic-specific attribute.

With this change we can switch to this contributed version of inferred spans and add the elastic.is_child attribute on the span link via the customization hook.