fix(server): Reorder DSC validation for correct client sample rate

[jan-auer]

This PR contains several fixes to the Envelope processor surrounding DSC
validation. Together, these fixes ensure that the _dsc metadata field
in the event payload contains the same values that were used during
sampling and normalization. It also fixes the trace.client_sample_rate
field in case a foreign DSC was reset.

Background

Relay validates whether the DSC originates in a project of the same
organization. If this is not the case and we were to use this DSC for
dynamic sampling, we would be applying sampling rules and rates from a
different organization.

If the DSC is invalid, we try to compute a new one from the information
in the event payload: We take the trace ID, environment, release, and
project key from event data. This simplifies the sampling routines,
since they can afterwards use the DSC and do not have to access the
event payload.

Fixes

1. If computing a fallback DSC from the event payload fails, remove the
   DSC from the event payload. Previously, we would leave the old DSC
   in.
2. Retain the client sampling rate from the original DSC. While the
   remaining DSC contents cannot be used, the sampling rate was actually
   applied and must be retained for extrapolation. Previously, we
   removed it along with the rest of the foreign DSC.
3. Validate the DSC before finalize_event, where it gets written into
   the event payload _dsc field. Since we validate the DSC both in
   PoPs and processing Relays, in practice the field always received the
   correct value. However, we now ensure correct order.
4. Validate the DSC before normalization, where we materialize the
   client sampling rate into the event's trace context. Previously,
   these two functions were switched and potentially wrong data was
   written.
5. Apply DSC validation in the errors pipeline so that the correct
   _dsc metadata is materialized there, too.