Report dropped spans in client reports

[buenaflor]

Description

Context: https://github.com/getsentry/projects/issues/70

Cases we need to implement this in Dart/Flutter:
All platforms

• sampling
• beforeSend / beforeTransaction
• event processors

Web, Linux, Windows:

• rate limiter
• network error

For iOS, macOS and Android the rate limiter and network error is implemented by the native sdks

### Fixed by
- [x] Cocoa
- [ ] https://github.com/getsentry/sentry-dart/pull/2154
- [ ] https://github.com/getsentry/sentry-dart/pull/2144