feat(user-agent-auth): add RSA and ECDSA auth key types

Extend user-agent authentication to support Ed25519, ECDSA (secp256k1), and RSA (PSS+SHA-256) with minimal protocol and storage changes. Add key_type to auth requests and useragent_client, update key parsing/signature verification paths, and keep backward compatibility by treating UNSPECIFIED as Ed25519.

server/crates/arbiter-server/Cargo.toml

@@ -42,7 +42,10 @@ argon2 = { version = "0.5.3", features = ["zeroize"] }
 restructed = "0.2.2"
 strum = { version = "0.27.2", features = ["derive"] }
 pem = "3.0.6"
-k256 = "0.13.4"
+k256.workspace = true
+rsa.workspace = true
+sha2.workspace = true
+spki.workspace = true
 alloy.workspace = true
 arbiter-tokens-registry.path = "../arbiter-tokens-registry"