fix: spec for list keys api functions webapp

[munishsuri]

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Response from Spec of List Keys currently points to String dictionary , which is not correct, it should be function keys , as depicted in the screenshot.

1. Changing the Spec to Correct Info
2. Creating a definition for function key

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API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
Java	com.azure.resourcemanager:azure-resourcemanager-appservice-generated
JavaScript	@azure/arm-appservice