App Config enhancements

[shanoor]

Additions:

• userassigned identies support
• custom private endpoint RG
• reworked key creation to use azurerm_app_configuration_key
• connection string added to outputs

PR Checklist

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• [x] I have added example(s) inside the [./examples/] folder
• [x] I have added the example(s) to the integration test list for normal (~30 minutes) or long runner >30 minutes
• [x] I have checked the coding conventions as per the wiki
• [x] I have checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Does this introduce a breaking change

• [ ] YES
• [x] NO

Testing