[Dynamic Sampling] Basic support

[lcian]

Description

We want the SDK to support Dynamic Sampling.
For this to happen, we need to:

• create the necessary data structures (DSC)
• collect the DSC from incoming requests
• send out the DSC when sending the envelopes to Sentry

As we are not instrumenting any outgoing network requests, we are not concerned with serializing the DSC into the baggage header or propagating it to other SDKs.

Spec