Create getAppMetadata handler in wrapper

[Quazia]

The correct RPC handlers need to be added in order to receive the getAppMetadata request from the API. This should take the form of a get-app-metadata.js file in aragon-wrapper/src/rpc/handlers/index.js. This function should only return data relevant to the application the request is coming from, so data should be filtered based on the to property of the addMetadata observable with respect to the application that initiated the request. This function must also be tested in the aragon-wrapper/src/rpc/handlers/ folder.