Add support for deferred executors. (#14859)

* Add support for deferred executors.

* More docs.

* Include from quantum.h

* Cleanup.

* Parameter checks

* Comments.

* qmk format-c

* I accidentally a few words.

* API name change.

* Apply suggestions from code review

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

* Review comments.

* qmk format-c

* Review comments.

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

quantum/deferred_exec.h

@@ -0,0 +1,38 @@
+// Copyright 2021 Nick Brassel (@tzarc)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// A token that can be used to cancel an existing deferred execution.
+typedef uint8_t deferred_token;
+#define INVALID_DEFERRED_TOKEN 0
+
+// Callback to execute.
+//  -- Parameter trigger_time: the intended trigger time to execute the callback -- equivalent time-space as timer_read32()
+//               cb_arg: the callback argument specified when enqueueing the deferred executor
+//  -- Return value: Non-zero re-queues the callback to execute after the returned number of milliseconds. Zero cancels repeated execution.
+typedef uint32_t (*deferred_exec_callback)(uint32_t trigger_time, void *cb_arg);
+
+// Configures the supplied deferred executor to be executed after the required number of milliseconds.
+//  -- Parameter delay_ms: the number of milliseconds before executing the callback
+//  --           callback: the executor to invoke
+//  --           cb_arg: the argument to pass to the executor, may be NULL if unused by the executor
+//  -- Return value: a token usable for cancellation, or INVALID_DEFERRED_TOKEN if an error occurred
+deferred_token defer_exec(uint32_t delay_ms, deferred_exec_callback callback, void *cb_arg);
+
+// Allows for extending the timeframe before an existing deferred execution is invoked.
+//  -- Parameter token: the returned value from defer_exec for the deferred execution you wish to extend.
+//  --           delay_ms: the new delay (with respect to the current time)
+//  -- Return value: if the token was found, and the delay was extended
+bool extend_deferred_exec(deferred_token token, uint32_t delay_ms);
+
+// Allows for cancellation of an existing deferred execution.
+//  -- Parameter token: the returned value from defer_exec for the deferred execution you wish to cancel.
+//  -- Return value: if the token was found, and the executor was cancelled
+bool cancel_deferred_exec(deferred_token token);
+
+// Forward declaration for the main loop in order to execute any deferred executors. Should not be invoked by keyboard/user code.
+void deferred_exec_task(void);