Refactor Envelope build & sending

## Summary
Currently, events are handed over to transport for sending. In order to fully switch to Envelopes, transport should not know about events and instead accept envelopes for sending. Transport should also not do any event-type specific processing - all the information transport needs is contained in the Envelope & its Items. See https://develop.sentry.dev/sdk/envelopes/

## Motivation
Besides finishing the move to Envelopes, the separation of concerns would allow us to build envelopes with multiple items, without hacking around events, as I've had to do in [here](https://github.com/getsentry/sentry-go/pull/626/files/8417a7bdf3462217a959ea9e1f4e2e0876dc235f#diff-5982a4ca350449c8a5deba675bc4bc4bb0f1823c45e6c7ba0076cc6b159fef8fR156) - adding a `transactionProfile` field to `Event` and handling that in `transport.go`.

## Additional Context
Ideally, I'd also suggest something to think about in the future - changing the APIs to not accept `Event` struct but rather an interface, sth. like which just knows how to serialize and deserialize itself. This would also make it more flexible and avoid adding fields specific to different event types, e.g. Transactions, as there already are.