feat(core): Add ignored client report event drop reason (#18815)

This PR adds `ignored` as a new allowed client report discard reason.
The new reason can already be used (see
[ticket](https://linear.app/getsentry/issue/FE-678/add-new-ignore-discard-reason-for-client-reports)
for details). Technically, we only need it right now for span-first to
record `ignoreSpans` hits. But this reason also meant for more telemetry
types, so we can merge it into develop already.

Author: Lms24

packages/core/src/types-hoist/clientreport.ts

@@ -9,7 +9,8 @@ export type EventDropReason =
   | 'sample_rate'
   | 'send_error'
   | 'internal_sdk_error'
-  | 'buffer_overflow';
+  | 'buffer_overflow'
+  | 'ignored';
 
 export type Outcome = {
   reason: EventDropReason;