Implement app streams: stdio for apps (#640)

Implement file IO for applications to facilitate text input/output capturing.

Apps can now launch apps and:
- Read their stdio output
- Write to their stdio input

Modules/app-module/source/null_device.cpp

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: Apache-2.0
+#include <app/private/null_device.h>
+
+namespace {
+
+ssize_t null_read(void*, void*, size_t) {
+    return 0; // EOF
+}
+
+ssize_t null_write(void*, const void* buffer, size_t size) {
+    (void)buffer;
+    return static_cast<ssize_t>(size); // discarded, reported as fully written
+}
+
+error_t null_close(void*) {
+    return ERROR_NONE;
+}
+
+error_t null_await(void*, AppFileWait, TickType_t) {
+    return ERROR_NONE; // always ready
+}
+
+uint32_t null_poll(void*) {
+    return APP_FILE_READABLE | APP_FILE_WRITABLE;
+}
+
+const AppFileOps NULL_OPS = {
+    .read = null_read,
+    .write = null_write,
+    .close = null_close,
+    .await = null_await,
+    .poll = null_poll,
+};
+
+constexpr AppFile NULL_FILE = { .ops = &NULL_OPS, .object = nullptr };
+
+} // namespace
+
+extern "C" {
+
+const AppFile* app_null_file(void) {
+    return &NULL_FILE;
+}
+
+} // extern "C"