Proof-of-work based signet faucet

Implementation
,
3

Thanks! This still works for me:

$ ~/src/bitcoin/contrib/signet/powcoins setup-wallet
$ ~/src/bitcoin/contrib/signet/powcoins claim --max-diff=25 mfqXWKj6cmRq18yH9pyrLMGY7L5QxAMHWR
INFO:root:sendrawtransaction result ce82bca0bfd81cacac0f7b108b20de5b888957ba516745f6acdea26244138817
$ ~/src/bitcoin/contrib/signet/powcoins claim --max-diff=16 mfqXWKj6cmRq18yH9pyrLMGY7L5QxAMHWR
ERROR:root:Faucet is too difficult (min difficulty 24)
$ ~/src/bitcoin/contrib/signet/powcoins claim --max-diff=24 mfqXWKj6cmRq18yH9pyrLMGY7L5QxAMHWR
INFO:root:sendrawtransaction result 9d3a5fd2f885b490ce60447321ecd9a379f7ac980e5b9ef218ac3f16e7d5a766

May those txids serve as timestamps here.