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 DiscountCT is available as our endpoints need to be used for lowball for now.

[kilrau]

Crucial for this issue is the case: boltz backend down and make uncooperative claims and refunds work via third party endpoints. Since cooperative path is always preferred we want to handle these similarly to boltz-client:
refunds: retries coop refund every block until timeout block height reached, then goes scriptpath
claims: tries coop 3 times (to account for connection flakiness), if these fail goes scriptpath

[kilrau]

This is currently blocked by lack of DiscountCT on Liquid mainnet, which we expect to be available in the next months. Then we can implement this.

[kilrau]

DiscountCT is available now.