Add Carthagenet to named chains.

There is a new test network corresponding to the next expected Tezos protocol proposal. Also added a couple of convenient functions for getting the name of a network based on its chain_id and vice versa.

The trouble appears to be in the Tezos.Common.NodeRPC.Sources module, where it expects both Obsidian and Blockscale to be running node-like services corresponding to each test network. However, in reality, those endpoints may not actually exist. The module will probably have to be overhauled to recognize this, and perhaps renamed/relocated as these endpoints it is referring to are not really Tezos nodes, but have other custom APIs.