[Feature Request]: Variable to add providers to default list of providers to register

### Which product(s) are you raising for?

Sub/LZ Vending (TF)

### What is the feature request?

Right now, you can either use the default list of providers from the vending module OR specify the entire list of providers that you want to register.
It would be nice if we could provide a list of additional providers to register to the default list instead of having to specify the entire list.
Default + List of additional providers

### Potential implementation/configuration

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Start by locating the Sub/LZ Vending Terraform module's configuration for the default provider list and the option for specifying the full list. Compare how those settings are documented and tested, then define done as allowing additional providers to be merged with the defaults without requiring the complete list.

Written by the indexing model from the issue text.