[Feature Request] Split into more crates to allow shared workflows between integ/replay tests

[Sushisource]

Right now there's no good way to define shared workflows in the test_utils crate because it depends on core in non-dev mode, but core depends on it in dev-mode, and thus if you try to use shared workflows in unit tests types mismatch and everything breaks.

Seemingly the only real way to fix this is to split out all the code that deals with mocking server responses and building fake histories into yet another crate (as well as possibly another crate for all the gateway stuff, possibly the pollers abstraction) so that we can define tests which can use mock responses and the history builder etc and also depend on the shared workflows.

Pretty big and annoying chunk of work but ultimately will result in a nicer structure

[Sushisource]

Almost certainly makes sense to just do #176 at the same time too