[Services-test/identity] Refactor create_device

Summary:
This function is useful for a lot of scenarios,
make into it's own function, to allow for it to be used
in many locations

In support for testing RefreshUserPreKey:

https://linear.app/comm/issue/ENG-3942

Depends on D8434

Test Plan:
Start docker, localstack, run terraform

cd services/commtest
cargo test --test identity_access_tokens_tests

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8463