Add `Outbound` Remote Signer implementation

[ViktorTigerstrom]

This PR introduces the functionality to utilize an alternative remote signer implementation, in which the remote signer node establishes an outbound connection to the watch-only node.

In the existing remote signer implementation, the watch-only node establishes an outbound connection to the remote signer, which accepts an inbound connection. The implementation introduced by this PR eliminates the need for the remote signer to allow any inbound connections.

To enable an outbound remote signer using the functionality introduced in this PR, please run make release-install & follow these steps: https://github.com/ViktorTigerstrom/lnd/blob/2024-05-add-outbound-remote-signer/docs/remote-signing.md#outbound-remote-signer-example

Note: This PR does not address the requirement for the remote signer to remain online while the watch-only node is operational. Currently, all RPC requests sent to the Remote signer will fail if it goes offline, and the health monitor will then proceed to shutdown the watch-only node. Additionally, this PR does not implement any validation on the remote signer side, i.e. the Remote Signer will blindly sign whatever is sent to it. These issues will be addressed in future PRs.

Final note: I plan resolve any CI issues ASAP, so reviewers can await those fixes before starting the review if preferred. I also intend to add some review comments on a few points where feedback would be particularly helpful.

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[ziggie1984]

Interesting PR 👀, does this somehow relate to the VLS project, a bit of time ago somebody told me that there is a proxy in the making to connect LND to the VLS signer project, wondering if there is some progress made in that direction ?

[ViktorTigerstrom]

Interesting PR 👀, does this somehow relate to the VLS project?

Thanks a lot! I would say that the current PR is somewhat related to VLS in terms of future functionality it could enable. However, this PR only focuses on reversing the connection setup between the watch-only node and the signer node, so that the signer node makes the outbound connection to the watch-only node instead of the other way around. In a setup where mobile signer wallets use VLS for validation which are connected to watch-only nodes, this functionality is a necessary pre-requisite unless the mobile wallets are online constantly.

However, this PR as it currently stand does not add any validation or enable an integration with VLS yet. The goal is to address such needs in future PRs. My hope is that the RemoteSignerClient introduced in this PR can be updated in the future to support such functionality, but this is still to be determined.

a bit of time ago somebody told me that there is a proxy in the making to connect LND to the VLS signer project.

I think the project you're referring to is: https://github.com/NYDIG/lndsigner