Include boltz regtest as submodule

[danielgranhao]

This PR replaces the existing regtest setup (that was a copy of boltz regtest) with using boltz regtest as a submodule. The motivation is letting us more easily keep up with changes to the boltz regtest setup. A short-term downside is that this will run some containers we don't need, but the solution is to propose upstream having those services be optionally disabled.

[dangeross]

I think if we add submodules we should also add notes (maybe in DEVELOPMENT/CONTRIBUTION) on how to clone the repo. Is it git clone --recurse-submodules --remote-submodules <repo-url>?

[danielgranhao]

I think if we add submodules we should also add notes (maybe in DEVELOPMENT/CONTRIBUTION) on how to clone the repo. Is it git clone --recurse-submodules --remote-submodules <repo-url>?

Given this submodule isn't essential, I think it's fine to add that note on the regtest README (added in b637db6). Let me know if you disagree.

Also, for context, I plan on improving regtest documentation on a later PR.

[dangeross]

There a specific reason for this change?

[danielgranhao]

It updates the data sync service to a more recent version (to include breez/data-sync#4). Once breez/data-sync#18 is merged, we can finally set it to the proper value. I've just pinged @hydra-yse to see if we can do it now.

[roeierez]

LGTM