feat: add typed constructors for AuthSingleSig

[giwaov]

Summary

Add scheme-specific constructors to AuthSingleSig that accept typed public keys directly, preventing mismatches between the key type and authentication scheme.

Rationale

The existing AuthSingleSig::new(pub_key_commitment, auth_scheme) constructor takes a generic PublicKeyCommitment and a separate AuthScheme, which allows callers to accidentally pair an ECDSA key commitment with AuthScheme::Falcon512Poseidon2 (or vice versa). Typed constructors eliminate this class of bug by deriving the scheme from the key type.

Changes

crates/miden-standards/src/account/auth/singlesig.rs:

• falcon512_poseidon2(pub_key): accepts a falcon512_poseidon2::PublicKey and creates the component with the correct scheme
• ecdsa_k256_keccak(pub_key): accepts an ecdsa_k256_keccak::PublicKey and creates the component with the correct scheme
• from_public_key(pub_key): accepts a PublicKey enum and extracts both the scheme and commitment

crates/miden-protocol/src/account/auth.rs:

• Add From<ecdsa_k256_keccak::PublicKey> for PublicKeyCommitment for parity with the existing Falcon512 conversion (required by the new constructors)

The existing new() constructor is preserved for backward compatibility.

Test plan

• cargo check -p miden-standards and cargo check -p miden-protocol both pass
• Existing tests continue to pass since the original new() constructor is unchanged
• The new constructors are thin wrappers over new() with type-safe inputs

Closes #2537