As a user I'd like to be able to save events for sending at a later time (but cannot due to marshalling issues)

### Problem Statement

We're using sentry in a cli application that is very short lived, so we want to save events to disk and then only send them to sentry when requested. That works fine for a single transaction today, but spans that are contained in a transaction fail, as the decoding/encoding is inconsistent (if you run it today it fails).

### Solution Brainstorm

Allow for encoding / decoding of transaction/span so that the user can delay sending events.

Alternately if this were internalized into some delayed queue that can be sent later, that works as well.