Implement Sampling Seed Propagation

[kahest]

Description

See https://github.com/getsentry/projects/issues/316

To increase the chance of capturing complete traces when users return a new sample rate tracesSampler in backend services, we propagate the random value used by the SDK for computing the sampling decision instead of creating a new random value in every service. Therefore, across a trace every SDK uses the same random value.

Develop: Spec

Similar to getsentry/sentry-react-native#4425 we most likely need to only bump the JS SDK.

See also

Sample implemeantion in PHP getsentry/sentry-php#1793.

[lucas-zimerman]

We will wait for an official V9 release since it was only implemented on it, if there is some urgency on it, we should port the change to V8.