[Premium] Periods are replaced with underscores in application settings keys

**Describe the Bug**
Using application string names in local.settings.json that contain periods works as expected. Creatingapplication string names in the portal with periods passes validation. When viewing the app string names in the portal the periods still appear in the name. However, attempting to access these application strings from the code during runtime fails with a key not found error. A runtime dump of all the application strings shows that the periods have been replaced with underscores. When I replaced the app string names periods with underscores in the code the app string name was found and returned the expected value

**To Reproduce**
1. Create an application string whose name contains periods.
2. Attempt to access those application string by referencing the name using periods.
3. A dump of the application strings shows that the periods in the key names have been replaced by underscores
4. Attempting to access those application strings by using the name with the periods replaced with underscores works

**Expected behavior**
When periods are used in the name of application strings they are able to be referenced by periods OR documentation and portal validation of application string names clearly explains that periods are replaced with underscores.

**Screenshots**
portal shows key name with periods

Dump of applications strings at runtime shows underscores have replaced periods