[client/rest] Can we add the database size in rest?

[AnthonyLaw]

Current Behaviour

We are unable to tell what is the database size in the current network. It will help node owners to plan for the storage size before they run a node, and information is also helpful for analytics and research, you want to map total state growth over time and cross-reference that with product releases.

Expected Behaviour

We can add that information to this endpoint /node/storage.