Allow the use of custom Ethereum JSON-RPC endpoint

[james-chf]

Closes #417 and #464

This PR makes it so that the ledger will not start up geth if the ledger.ethereum.mode = "Remote". The motivation for this PR is to be able to test the ledger more easily (with a separately managed geth process or some standin like npx hardhat node, or some remote Ethereum endpoint). I've manually tested using a separate geth process and passing envvars to override the appropriate config e.g.

# with npx hardhat node
export ANOMA_LEDGER__ETHEREUM__MODE='Remote'
target/debug/namadan ledger run

[james-chf]

I'll make the following changes to this PR:

• use a struct (e.g. host + port) rather than a string for configuring the custom RPC endpoint
• this should be set in Config, not in an envvar

[james-chf]

I thought about the issue of oracle_rpc_endpoint potentially containing a secret API key, like in the case where someone was using an Alchemy endpoint e.g. https://eth-goerli.alchemyapi.io/v2/${ALCHEMY_API_KEY}. Generally I don't think this config value should be treated as secret, so it's still OK to log oracle_rpc_endpoint as we're currently doing. I don't think having sensitive information in an (Ethereum endpoint) URL is a good practice we need to accommodate.