Add private-key-knowledge VSS authentication

[tankyleo]

    Add private-key-knowledge VSS authentication

    In PR https://github.com/lightningdevkit/ldk-node/pull/755, we shipped
    a VSS authentication scheme in ldk-node that grants access to data
    stored in VSS based on the knowledge of a private key. This
    authentication scheme is the default method of authentication to VSS
    server in ldk-node.

    Here we wire up this authentication scheme to the public API of this
    crate.

    With this commit, if only the `--vss-url` argument is passed to the cli
    example, with no other VSS arguments, sigs auth is now used. VSS server
    default builds expect this authentication mechanism, and it works out of
    the box, so it is very demo-friendly.

    First launch VSS server:

    `cargo run -- server/vss-server-config.toml`

    then launch the cli example against VSS:

    `cargo run -- --vss-url http://127.0.0.1:8080/vss`

    Commit written with codex.

[benthecarman]

lgtm!