Add `before_send_feedback` hook

[limbonaut]

Considering that user feedback events will be enriched with scope data, and they are not supposed to go through before_send, we should add respective before_send_feedback hook.

From spec:

SDKs SHOULD implement a beforeSendFeedback callback to allow users to modify or discard the feedback before it is sent. This callback SHOULD be similar to the existing beforeSend callback used for error events. Feedback events MUST NOT go through the beforeSend hook.

[linear-code]

NATIVE-207