Add a simple public-key-based authenticator

[TheBlueMatt]

JWT-based authentication is currently largely the default due to
its integration in `ldk-node` indirectly via LNURL-auth. This is
great, but massively over-engineered (and requiring yet another
service devs have to set up and maintain) for just authenticating
to a storage service (and maybe an LSP).

Here we add a much simpler authentication scheme, based simply on
proof-of-knowledge of a private key. This allows for a simple VSS
install without requiring any additional services. It relies on
some higher-level authentication to limit new account registration,
but that can be accomplished through more traditional anti-DoS
systems like Apple DeviceCheck.

[ldk-reviews-bot]

👋 I see @valentinewallace was un-assigned.
If you'd like another reviewer assignment, please click here.

[tnull]

Here we add a much simpler authentication scheme, based simply on
proof-of-knowledge of a private key.

I'm a bit skeptical of this approach right now. Generally there is no harm in supporting different authentication schemes, but we so far failed to establish a single standard authentication mechanism.

For that reason I'm afraid that adding more different authentication schemes could result in an even more complicated/confusion support matrix for users.

Any thoughts on that? Wouldn't it be preferable to commit to one scheme for now to finally establish a standard? Could you expand on why you couldn't just use the existing JWT auth for your usecase?