Create a funding service

Now that fees are enabled, things are getting difficult to test without having the faucet available.

Moreover, on `mainnet` we won't have free access to funds from the faucet at all, so we also need a solution for this. The amount of funds will be negligible since txs are "cheap".

Create a service which has an endpoint to request funds for a specific account. e.g. `Request { account_id, amount }`. This triggers creating a tx to create a private p2id note for that account. Respond with the note ID, ideally with the inclusion proof aka wait for it to be committed.

Infrastructure accounts will periodically request funds to top themselves up i.e. this is not done frequently. This means there is no real benefit in attempting to coalesce requests into a single transaction. Though perhaps this would occur e.g. at monitor start up. Still this is an optimisation, not a requirement.

Authentication would be restricted at the infrastructure level, i.e. no need to bake in auth methods into the service.

The service should assume it has funds for now; initially we can prefund/create this at genesis.

Consider adding a maximum amount via cli.