Add a best-effort method for estimating liquidity fees

[pm47]

Returns a best-effort estimate of liquidity fees for a given amount.

Note that:

• it depends on the current mining feerate, which is volatile
• this estimate is the full cost, it doesn't take into account the fee credit (which will be deducted)
• if you receive A sat with a setting of --auto-liquidity B, then the actual liquidity amount will be A+B sat.

./phoenix-cli estimateliquidityfees --amountSat 2000000
{
    "miningFeeSat": 1219,
    "serviceFeeSat": 20000
}

See #88,#104.

[t-bast]

LGTM 👍

Maybe it could be useful to also optionally provide a feerate to override the one used by phoenixd? Since the feerate that phoenixd uses internally is not the feerate that will be used for pay-to-open (it will be a feerate decided by the LSP)? Or we don't care for now since this will get better with the new on-the-fly funding, at which point we will know the feerate that will be used?

[pm47]

Since the feerate that phoenixd uses internally is not the feerate that will be used for pay-to-open (it will be a feerate decided by the LSP)? Or we don't care for now since this will get better with the new on-the-fly funding, at which point we will know the feerate that will be used?

It's not the same because of sampling frequency, but it's the same source (phoenix gets its feerate from mempool.space, not electrum). I'd keep things simple, waiting for the new on-the-fly.