feat(network-monitor): fund accounts from the funding service

[SantiagoPittella]

Summary

Adds support in hte network monitor for the funding service. The chain's faucet is no longer a source of fees, because a network does not always run a public faucet. --faucet-url still configures the faucet checks.

--funding-request-timeout defaults to two minutes, because the service answers only once its note is committed.

Added operator documentation and internal page for design.

Changelog

[[entry]]
scope       = "network-monitor"
impact      = "breaking"
description = "Fees are funded from the funding service via `--funding-service-url`; the faucet is no longer a source of fees."

[[entry]]
scope       = "docs"
impact      = "added"
description = "Operator documentation for the funding service."

[Mirko-von-Leipzig]

Not this PRs problem, but I wonder if we can't somehow make it so that these things are always required, even on zero-fee chains.

One option is of course to just not have zero fee chains.. but I was hoping the flows could remain agnostic to the fee amount, as in fee=N holds even for N=0.