Introduce the equivilant of TraceContinuationStrategy

[stevejgordon]

We should replicate (as closely as possible) the TraceContinuationStrategy feature from the existing APM agent. We may need to clone the activity with span links to achieve this. We should consider extending this configuration to allow setting a strategy applied by a filter such that different endpoints can have different behaviours. Or we could reintroduce the "es" trace state to play nicely with the existing APM Agent.