Add proto definitions for exec output options

When running an exec, we block the process if its stdout or stderr has
not been consumed. This can be surprising behavior because if a user
blindly execs, their process could hang without them knowing. This
commit adds proto definitions enabling subprocess-like selection of
output style.

modal_proto/api.proto

@@ -111,6 +111,13 @@ enum DeploymentNamespace {
   DEPLOYMENT_NAMESPACE_GLOBAL = 3;
 }
 
+enum ExecOutputOption {
+  EXEC_OUTPUT_OPTION_UNSPECIFIED = 0;
+  EXEC_OUTPUT_OPTION_DEVNULL = 1;
+  EXEC_OUTPUT_OPTION_PIPE = 2;
+  EXEC_OUTPUT_OPTION_STDOUT = 3;
+}
+
 enum FileDescriptor {
   FILE_DESCRIPTOR_UNSPECIFIED = 0;
   FILE_DESCRIPTOR_STDOUT = 1;
@@ -703,6 +710,8 @@ message ContainerExecRequest {
   // Send SIGTERM to running container on exit of exec command.
   bool terminate_container_on_exit = 4;
   bool runtime_debug = 5; // For internal debugging use only.
+  ExecOutputOption stdout_output = 6;
+  ExecOutputOption stderr_output = 7;
 }
 
 message ContainerExecResponse {