fix(pii): Scrub user fields in span.sentry_tags (#4364)

User fields are transferred into `span.sentry_tags` before PII
scrubbing. Most user fields are marked as `pii = true`, that is, they
should be scrubbed by default.

This PR

1. converts `sentry_tags` into a special type `SentryTags`, such that we
   can mark specific tags as `pii = true`,
2. adds `sentry_tags.user_ip` to the builtin PII selector for IP
   scrubbing.

CHANGELOG.md

@@ -12,6 +12,7 @@
 - Accept incoming requests even if there was an error fetching their project config. ([#4140](https://github.com/getsentry/relay/pull/4140))
 - Rate limit profiles when transaction was sampled. ([#4195](https://github.com/getsentry/relay/pull/4195))
 - Fix scrubbing user paths in minidump debug module names. ([#4351](https://github.com/getsentry/relay/pull/4351))
+- Scrub user fields in span.sentry_tags. ([#4364](https://github.com/getsentry/relay/pull/4364))
 
 **Features**: