Add optional 3rd party endpoints for tx broadcasting and reading the chain

[kilrau]

We want to enable users to broadcast and read the chain via 3rd party endpoints. But @michael1011 brought up a valid point as where some users might not want their transaction info to be sent to a third party and prefer to still broadcast via Boltz, so we probably want to allow changing endpoints via settings menu but stick with ours by default.

Options in settings menu:

1. Boltz
2. mempool.space
3. blockstream.info

CORS might be an issue as always. And we can't really do that before we have cheapCT as our endpoints need to be used for lowball for now.