add "Demo Exchange" to LightningBitcoinExtension

[johnzweng]

Description:

To be able to test also with dummy exchange, I added Dummy Exchange support for currency LBTC

Sidenote: was created to test this issue here: https://generalbytes.atlassian.net/browse/BPUB-1895

[pvyhnal-generalbytes]

note we'll have to fix BPUB-1895 before merging this

[johnzweng]

note we'll have to fix BPUB-1895 before merging this

@pvyhnal-generalbytes technically: Not necessarily. 🙂 By just merging this, nothing will change. BPUB-1895 describes an error which happens with any LBTC exchange. As soon as any exchange for LBTC is configured, the problem will occur.

This PR here just adds a Demo Exchange, which does nothing and shouldn't be used in production. But of course I understand your caution. As soon as there is a LBTC exchange available in the dropdown menu (and even if it is this Demo Exchange), people might configure it and run into the BPUB-1895 problem (which was reported by me).

So I'm totally fine if you wait for BPUB-1895 before merging this (I just added it to a custom extension for my tests). 🙂