Shared/Central parameters support

[Jefajers]

This issue is a split from #831

Ability to associate/import/refence multiple parameter files into one outcome.

A fair amount of parameters will be shared by customers as well, which could lead to duplication among the parameter files. However, the Bicep community is already working on parameter imports in Azure/bicep#12019.

[schaijkw]

A cascading style will be nice. So you can inherit from a more general bicepparameter file and still have the ability to overrule an imported parameter. Like the union() function currently works. The last written value survives.