[FEATURE REQ] Support Parameters "allowedValues" property

### Please describe the feature or suggestion.

Template parameters can have an array defined specifying what values are allowed for the parameter.
https://learn.microsoft.com/en-us/azure/azure-resource-manager/templates/parameters#allowed-values

Currently, if a parameter has allowed values specified, template parsing (without using a parameters file) throws an exception because a mock value is used that (very likely) will not be an allowed value.

BPA should be able to pick a value from this list and use that for the parameter value instead of making up a value on its own.

### Additional context

This could also be considered a bug, but I'm filing the issue as a feature request.