The new SDK should only send envelopes vs. send events /store/.

https://develop.sentry.dev/sdk/envelopes/

So we should only send things to /envelopes/, we can even discontinue the send_event function on the Transport and only have send_envelope.

So we need to intro a Envelope class with a few helpers:

env = Envelope::from_event()

env = Envelope::from_transaction()