feat(auth): add protobuf definitions for AuthService

protobufs/auth.proto

@@ -0,0 +1,32 @@
+syntax = "proto3";
+
+package arbiter.auth;
+
+import "google/protobuf/timestamp.proto";
+
+message AuthChallengeRequest {
+    bytes pubkey = 1;
+}
+
+message AuthChallenge {
+    bytes pubkey = 1;
+    bytes nonce = 2;
+    google.protobuf.Timestamp minted = 3; 
+}
+
+message AuthChallengeSolution {
+    AuthChallenge challenge = 1;
+    bytes signature = 2;
+}
+
+message AuthResponse {
+    string token = 1;
+    string refresh_token = 2;
+    google.protobuf.Timestamp expires_at = 3;
+    google.protobuf.Timestamp refresh_expires_at = 4;
+}
+
+service AuthService {
+    rpc GetChallenge(AuthChallengeRequest) returns (AuthChallenge);
+    rpc SubmitSolution(AuthChallengeSolution) returns (AuthResponse);
+}