Add strict mode option for parsing of JSON values

Currently, if the provider attempts to parse an invalid JSON key-value with content type `application/json`, it will default to interpreting the key-value as a string. However, this also means there are no exceptions thrown or logging to indicate that the key-value had an invalid JSON and that the key-value now contains a string.

A potential solution is to add an option to the provider that can be enabled to either log a warning or throw an exception when loading a key-value if it can't be parsed as a JSON and has the "application/json" content type. This would be added to the `AzureAppConfigurationOptions` class as a boolean proprety.

@zhenlan @jimmyca15 @avanigupta