Add a method to validate email

Signed-off-by: Nikolai Rodionov <iam@allanger.xyz>

proto/accounts/v1/accounts_v1.proto

@@ -12,6 +12,8 @@ service PublicAccountsService {
   rpc SignIn (SignInRequest) returns (SignInResponse) {}
   // Create a new account
   rpc SignUp (SignUpRequest) returns (SignUpResponse) {}
+  // Check whether a email is already in use
+  rpc IsEmailUsed(IsEmailUsedRequest) returns (IsEmailUsedResponse) {}
 }
 
 service AccountsService {
@@ -100,3 +102,11 @@ message RefreshSessionResponse {
 message RemoveSessionRequest {
   string refresh_token = 1;
 }
+
+message IsEmailUsedRequest {
+  string email = 1;
+}
+
+message IsEmailUsedResponse {
+  bool used = 1;
+}