Endpoint in API to indicate ChainId

[lullis]

Abstract

The API provides a way for to know the raiden address, but it does not indicate what chain (or chains?) it is connected to. This is not so much of an issue when Raiden was only deployed on ethereum mainnet, but with the plans to deploy it on Arbitrum it might be important

Motivation

One can think of applications that need to interact with multiple instances of Raiden nodes, and quickly figure out through the REST API what chain a particular instance is connected to.

Examples:

• a payment gateway or centralized exchange that wants to give the ability to make payouts to its users via Raiden
• Someone providing a mainnet -> arbitrum token bridge.

Specification

There should be an endpoint in the REST API that responds with the id of the chain that the raiden node is connected to. This can be a simple relay of the chainId value provided by the ethereum client.