Use subaccount model in create API response

[svenluijten]

This PR ensures the created subaccount is available to consumers of this API without having to make a new request. According to the docs, the POST /v5/accounts/subaccounts endpoint returns the created subaccount, so we'll use the already existing model in the project to return it.

[svenluijten]

It should be noted that this is a breaking change, as the $message property was previously filled with the response. But it made no sense that I had to call ->getMessage() to get the response body. I figured a separate ->getItem() method that returns the domain model works better.