webdav: add some credential feature

[Tofee]

Currently, when rustic webdav is started, the webdav access is open to everyone.

It would be nice to have an option to protect this access with a login/passowrd. We could even think of a simple protection, with the login begin whatever, and the password the same as the repository password.

What is your view on this?

[simonsan]

I think this is related to adding TLS support for webdav. If we implement a simple user/password authentication, this doesn't help much if on your intranet others can see the traffic and just read the credentials.

But anyway, webdav is done by the warp crate (which we might refactor to axum at some point) at the moment - if anyone wants to add improvements, they will be very local at the place where warp is called.

Originally posted by @aawsome in #1271 (comment)