Ability to mutate trace state for custom sampling

[connectwithnara]

Use case:
In order to implement one or more custom sampling strategies. For example - control service participation in a trace. Imagine the following topology.

A -> B -> C -> D

• Service A doesn't sample the incoming request however the trace context is initialized and sent downstream with sampling decision set to false.
• Service B is configured to start a trace for all (100% sampling) the requests matching the path /foo. It encodes this sampling decision in the trace state so any downstream services can participate.
• Service C doesn't do anything but propagates the context down.
• Service D is configured to participate in the trace initiated by Service B. It looks up the trace state and makes a sampling decision to record the spans.

Basically the ask is to provide hooks/API changes to support such a use case. Specifically, in this example service B will need access to modify trace state where the sampling decision is taken. Service D will need access to the trace state to make a sampling decision.

@bogdandrutu @tedsuo @anuraaga

[bogdandrutu]

Duplicate of #856, copied the example there.