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.

protobufs/user_agent.proto

@@ -5,9 +5,17 @@ package arbiter.user_agent;
 import "google/protobuf/empty.proto";
 import "evm.proto";
 
+enum KeyType {
+  KEY_TYPE_UNSPECIFIED = 0;
+  KEY_TYPE_ED25519 = 1;
+  KEY_TYPE_ECDSA_SECP256K1 = 2;
+  KEY_TYPE_RSA = 3;
+}
+
 message AuthChallengeRequest {
   bytes pubkey = 1;
   optional string bootstrap_token = 2;
+  KeyType key_type = 3;
 }
 
 message AuthChallenge {