Triggering synchronization is something that takes place locally on the Azure AD Connect server (and not with the Az PowerShell module but with the ADSync module).
There is no REST API on the Azure AD Connect server to allow you doing that. However, if the challenge is to trigger remotely, you can use remote PowerShell to do it from another system.
Unless REST API is a must... But then, what is the use case? I am curious :)