Add native support for liquidtestnet

[psgreco]

Currently using liquidtestnet requires a big config file, which is error-prone and uncomfortable for users, this MR adds support for just using -chain=liquidtestnet.

There are a few things that need to be discussed/addressed

1. Main port (currently set to 18891 because that's the one used originally, and the one that's configured in liquidtestnet.com)
2. RPC and extra ports, I just chose some non-overlapping ports that are "close" to the ones in the other liquid/elements networks.
3. Icon color, I just used something different than liquidv1 and elementsregtest, suggestions accepted 😉
4. liquid-testnet.blockstream.com is still using port 18892, it will be changed to 18891 soon.
5. both seeds are just CNAME dns records at the moment, they should be changed to real seed nodes (no changes in code required, but important for review)

[delta1]

adds support for just using -chain=elementsregtest

should this be -chain=liquidtestnet ?

[delta1]

partially tested ACK 3ccccca

I compiled elementsd and ran it with -chain=liquidtestnet, the daemon connected and started synchronizing the liquid testnet block headers

[psgreco]

adds support for just using -chain=elementsregtest

should this be -chain=liquidtestnet ?

Yes, fixed

[delta1]

ACK c6e074e

Tested by compiling the codebase, started elementsd -chain=liquidtestnet sync, and started elements-qt -chain=liquidtestnet sync successfully

[wintercooled]

ACK c6e074e

Compiled, installed and ran elements-qt with -chain=liquidtestnet and could sync to tip.

Note: I only tested running without -chain=liquidtestnet briefly - did not wait to sync live to tip.

[gwillen]

utACK. My main concern is making sure there is absolutely no accidental behavior change for existing users of custom chains. (As far as I can tell there is not.)

[apoelstra]

utACK c6e074e