Create explicit constructors for `AuthSingleSig`

[mmagician]

If this is such a common pattern, should we maybe make this easier?

impl AuthSingleSig {
    fn falcon_512(pub_key: Falcon512PublicKey) -> Self { ... }
}

Though I don't have enough insight into how these various components play together to really know what the right API is.

Originally posted by @Mirko-von-Leipzig in 0xMiden/node#1724 (comment)

[swaploard]

@bobbinth Can I take this issue?

[bobbinth]

@swaploard - it is all yours!