Reland "[Extensions] Implement sidePanel.onOpened"

This is a reland of commit 3ae67862081e121b7811095fa55abe4b3592699d

The original commit was reverted in
https://chromium-review.googlesource.com/c/chromium/src/+/6867414
due to a failing browser test:
  ExtensionOpenSidePanelBrowserTest.OpenSidePanel_OpenContextualPanelInActiveTab

After discussion, it was determined that this change was not the root
cause of the test failure, which is now believed to have been a flake.
This commit relands the original CL to proceed with the feature
implementation.

Original change's description:
> [Extensions] Implement sidePanel.onOpened
>
> Add the chrome.sidePanel.onOpened event. This event is triggered
> whenever the side panel is opened, in all scenarios. Returns tabId
> (only for tab-specific panels), windowId and path.
>
> Reference: https://github.com/w3c/webextensions/pull/779
>
> Bug: 434215968
> Change-Id: I2911b653f1078c770a7c1f140ba79cd9a0e82124
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6656092
> Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
> Reviewed-by: Oliver Dunk <oliverdunk@chromium.org>
> Commit-Queue: Oliver Dunk <oliverdunk@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1502167}

Bug: 434215968
Change-Id: I52ab7eea1bacf290207b220432873e416a073bcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6881067
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Commit-Queue: Harsh Singh <harshsinghiitism@gmail.com>
Reviewed-by: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Solomon Kinard <solomonkinard@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506083}