Add support for signet

[Amperstrand]

Tested with

lightningd --network=signet --log-level=debug --disable-plugin bcli  --plugin ~/plugins/sauron/sauron.py --sauron-api-endpoint https://mutinynet.com/api

Should also work for https://mempool.space/signet/api

[chrisguida]

@Mergifyio rebase

[mergify]

rebase

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[chrisguida]

@Mergifyio rebase

[mergify]

rebase

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[chrisguida]

@Kodylow was the last person to maintain sauron, I spoke to him and he says trustedcoin and futhark may be better.

Also he has a newer repo here that includes a go version and more stuff: https://github.com/Kodylow/sauron

@cdecker should we just merge this in or should we update sauron to point somewhere else?

Also it seems like a test for the new functionality would be great to have?

[cdecker]

Rebased on top of master.

[cdecker]

@Mergifyio rebase

[mergify]

rebase

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