OF-3188: Add ExtendedDiscoInfoProvider to provide generic support for XEP-0128
ExtendedDiscoInfoProviders can be registered with IQDiscoInfoHandler, and each will be called on disco requests to potentially add forms or form fields to the response.
OF-3188: Move methods up to IQDiscoInfoHandler, and call from handleIQ
This means that we'll see requests to Conference, PubSub, etc be routed through this method. It adds a domain to the method signature in ExtendedDiscoInfoProvider.getExtendedInfos to allow an ExtendedDiscoInfoProvider to apply domain-specific logic.
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