[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)
  • DSC added in https://github.com/getsentry/sentry-rust/pull/867/files and TransactionMetadata in a follow-up PR to that one. Maybe we'll need more.
• collect the DSC from incoming requests
• send out the DSC when sending the envelopes to Sentry
  • we needed to send out part of it to solve an issue with metrics extrapolation, as the backend would assume an incorrect sample rate when it was not sent out from the SDK feat(core): add some DSC fields to transaction envelope headers #869

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