Transport#send(SentryEnvelope) support for Hybrid SDKs #4417

The RN SDK uses the JS SDK for capturing and processing events, and if the Native layer is disabled, even the Transport.
We'd like to implement the User feedback and Attachments features (likely more features that depend on envelopes too, e.g. client reports) on the RN SDK but that is blocked since the transport only accepts an Event.

There should be a way to also customize theSentryEnvelope, since we'd need to read the RN Scope to get the attachments and pack it into the envelope before calling the transport.

Transactions are already sent as envelopes, but the difference is a Transaction is an Event with a different type + extra fields, so that worked just fine, since Attachments and User feedback is a totally different data structure/different envelope item, the way how it just does not work.

Issues: getsentry/sentry-react-native#1327 and getsentry/sentry-react-native#500

This issue is related to #4240

An option would be to override everything on the RN SDK, but since the JS SDK will or can offer proper support for it, it does not make sense for us to make it, this would duplicate code, more to maintain, and likely a new source of bugs, hence we'd like to get proper support for Envelopes.