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-useragent/Cargo.toml

@@ -14,6 +14,11 @@ tracing.workspace = true
 ed25519-dalek.workspace = true
 smlang.workspace = true
 x25519-dalek.workspace = true
+k256.workspace = true
+rsa.workspace = true
+sha2.workspace = true
+spki.workspace = true
+rand.workspace = true
 thiserror.workspace = true
 tokio-stream.workspace = true
 http = "1.4.0"