Add unique_string function

[smokedlinq]

This adds the ARM function uniqueString so that the community can migrate from existing ARM/BICEP based deployments that utilize the function to create resources.

The golang version was converted from a C#/PowerShell utility I wrote a few years ago.

The values can be verified with a simple bicep configuration such as:

output value string = uniqueString('/subscriptions/00000000-0000-0000-0000-000000000000', 'resource-id')

P.S. Without a spreadable tfsdk type for a list of strings, an array seemed to be the most appropriate approach as the parameter.