Support extended mempool.space API

[notmandatory]

This issue was triggered by an discord discussion with @wiz.

The mempool.space project provides an extended API on top of the blockstream electrs server API. In particular it offers improved fee and mempool data. The public mempool.space service could also be used in bdk documentation and testing, the service currently supports mainnet and testnet, and in the future will support signet.

Onion endpoints can be found with these commands:

curl -si https://mempool.space|grep -i onion        
onion-location: http://mempoolhqx4isw62xs7abwphsq7ldayuidyx2v2oethdhhj6mlo2r6ad.onion/

curl -si https://mempool.space/testnet | grep -i onion
onion-location: http://mempoolhqx4isw62xs7abwphsq7ldayuidyx2v2oethdhhj6mlo2r6ad.onion/testnet

[PatrickZGW]

Mempool.space also supports signet now :+1:

[wiz]

Please let us know if there's anything we can do to support you!

[notmandatory]

This could be solved by providing documentation and examples as described in bitcoindevkit/bdk_wallet#140 .