Add whenPromiseSettles function

Copied from the tests. We’ll use it for bridging between promise-using
methods and callback-using methods as we start changing the codebase to
use promises internally.

src/common/lib/util/utils.ts

@@ -452,6 +452,22 @@ export function promisify<T>(ob: Record<string, any>, fnName: string, args: IArg
   });
 }
 
+/**
+ * Uses a callback to communicate the result of a `Promise`. The first argument passed to the callback will be either an error (when the promise is rejected) or `null` (when the promise is fulfilled). In the case where the promise is fulfilled, the resulting value will be passed to the callback as a second argument.
+ */
+export function whenPromiseSettles<T, E = unknown>(
+  promise: Promise<T>,
+  callback?: (err: E | null, result?: T) => void
+) {
+  promise
+    .then((result) => {
+      callback?.(null, result);
+    })
+    .catch((err) => {
+      callback?.(err);
+    });
+}
+
 export function decodeBody<T>(body: unknown, MsgPack: MsgPack | null, format?: Format | null): T {
   if (format == 'msgpack') {
     if (!MsgPack) {