Consider adding support for hierarchical configuration in local.settings.json

We are planning to use a hierarchical configuration to support different modes of connecting to the service by having.

This is the current shape of settings:

``` json
{
"Values": {
"StorageConnection": "UseDevelopmentStorage=true",
}
}
```

This is what we want to support for AAD auth.
``` json
{
"Values": {
"StorageConnection": {
"endpoint": "https://pakrym0test0storage.blob.core.windows.net/",
}
}
}
```

Considering that functions runtime uses `IConfiguration` that already supports hierarchical data but we need to make sure values gets there in the correct shape. `AppSettingsFile` would need to change to support this.

Setting reading would need to change
https://github.com/Azure/azure-functions-core-tools/blob/28be3f19626161c5cca94d2d4376a2a3350c923b/src/Azure.Functions.Cli/Common/AppSettingsFile.cs#L11

Currently, it's possible to work around the absence of the feature by using the ASP.NET Core double underscore syntax (https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/?view=aspnetcore-3.1#environment-variables)

``` json
{
"Values": {
"StorageConnection__endpoint": "https://pakrym0test0storage.blob.core.windows.net/"
}
}
```

cc @fabiocav @mattchenderson @paulbatum

Start with src/Azure.Functions.Cli/Common/AppSettingsFile.cs at the linked setting-reading code, then compare the flat and nested local.settings.json examples. Verify how hierarchical values should reach IConfiguration, including the AAD endpoint shape, and consider the existing double-underscore workaround as a compatibility reference. Done means nested Values entries are read in the intended shape.

Written by the indexing model from the issue text.