Allow specifying an alternative fee endpoint like mempool.space

[sethforprivacy]

While testing out Join Market via JAM, I ran into a (common) occurrence of Core's estimatesmartfee endpoint being WAY off in it's estimate:

Using bitcoin network feerate for 3 block confirmation target (randomized for privacy): 129639 sat/kvB (129.6 sat/vB)

While fees were at 10-12sats/vb, JM got an estimate of ~129sats/vb, overspending by 10x even though it was supposed to be doing the default 3-block estimate. Of course this could be 20% lower/higher than the fee estimate it received, but even then is egregiously high. This is a common and well-known issue with Bitcoin Core, and is normally solved by using a more smart fee estimation system like that of mempool.space. As mempool.space has a Tor endpoint for fees, is easy to self-host, and widely used in the ecosystem, that would be a fantastic place to start allowing users to specify in the config file if they prefer it over Bitcoin Core.

The fee API can be found here: https://mempool.space/api/v1/fees/recommended