[Client, node, cf-core] Support any EVM chains and multiple chains concurrently

[ArjunBhuptani]

There's only a few relatively minor tweaks that are needed to make indra EVM chain-agnostic. This will help immensely not only with our own deployments (rinkeby and mainnet can be the same core node but with different chainId's in the Dai Card UI), but will also make it very simple to use Connext anywhere.

• Complete existing work on domain separators in contract commitments (part of audit fixes in issue [contracts] Audit tasks #601 )
• Add domain separator (particularly chainId) to all protocol messages and commitments
• Create a providerIndex.json file which can be configured on start. Each chainProvider here will be indexed by chainId
• Modify onchainTransaction service in node to get chainId from the State Channel params, look up the relevant provider from providerIndex and submit the transaction using that provider
• Modify client to take in chainId (which must be of the same chain as inputted ethprovider) on connect(). If a channel with that chainId + xpub combo has not been created, then it should setup. This can default to 1 for Ethereum
• Modify stores to index all channels by a combination of chainId + multisig?

[bohendo]

Create a providerIndex.json file which can be configured on start. Each chainProvider here will be indexed by chainId

Seems like this should contain a mapping from chainId -> ethProviderUrl. This'll be pretty simple & small, we might want to just use an env var for this eg:

INDRA_CHAIN_PROVIDERS='{"1":"mainnet.alchemy.io/abc123","4":"rinkeby.infura.io/abc123"}' make start-prod

[ArjunBhuptani]

Done!