Implement EventScrubber for PII scrubbing

[tonyo]

Summary

Implement an event scrubber and scrub events before sending them to the server when SendDefaultPII is set to false (which is the default).

Motivation

Various event fields (request headers, request body, breadcrumbs, etc.) can be a source of PII. We should be more transparent and flexible about what data is stripped, in what cases, and how.

The respective RFC has been approved, and Python already has a reference implementation.

Additional Context

The issue of PII stripping has been already brought up multiple times: #248, #485

Related RFC: https://github.com/getsentry/rfcs/blob/main/text/0070-document-sensitive-data-collected.md

Also related to #169