feat(data_source): added default_user data source to fetch data from configured provider

[lukas8219]

Feature

New dataSource to fetch the configured provider Username.

Why

Since it's not possible to do natively in Terraform something like rabbitmq.username it makes it harder to modularize default configurations or becoems cumbersome to require adding a variable to injected in the module just to use the default user.
Defining this data source makes it possible to move this logic to inside a child module.

Notes

I did try to re-use the dataSourcesReadUser function but I was having this panic: interface conversion: interface {} is nil, not string. An easy workaround is to simply use the data source to fetch the info