Add variables to ThrowError RPC

[jwulf]

gRPC documentation: https://docs.camunda.io/docs/apis-tools/grpc/#throwerror-rpc

message ThrowErrorRequest {
  // the unique job identifier, as obtained when activating the job
  int64 jobKey = 1;
  // the error code that will be matched with an error catch event
  string errorCode = 2;
  // an optional error message that provides additional context
  string errorMessage = 3;
  // JSON document that will instantiate the variables at the local scope of the
  // error catch event that catches the thrown error; it must be a JSON object, as variables will be mapped in a
  // key-value fashion. e.g. { "a": 1, "b": 2 } will create two variables, named "a" and
  // "b" respectively, with their associated values. [{ "a": 1, "b": 2 }] would not be a
  // valid argument, as the root of the JSON document is an array and not an object.
  string variables = 4;
}