node: introduce an endpoint to retrieve the current circulating supply

[Neotamandua]

Summary

Add a Rusk API endpoint that returns the current circulating supply of Dusk, without requiring clients to recompute it by replaying block rewards from genesis.

Detailed Description

The circulating supply can be derived by cumulatively summing block rewards starting from the genesis supply.

This approach is inefficient and any (archive) node could maintain this number more readily available.

Possible Solution

• Introduce a read-only endpoint (e.g. GET /supply/current).
• Introduce a gql endpoint that provides further supplyInfo with subfields for total supply, initial supply, circulating supply