[Extensions MV2] Expose "AllowLegacyMV2Extensions" in chrome://flags

Chrome has announced that MV2 extensions will gradually no longer be
supported in the browser. See more information at
https://developer.chrome.com/blog/resuming-the-transition-to-mv3.

In the next experiment phase, extensions will be disabled and may not
be re-enabled by the user. For a period of time, enterprises will
still be able to override this with the
ExtensionManifestV2Availability policy.

While MV2 extensions are supported in any capacity (including with the
enterprise policy), we need to make sure developers have a way to load,
run, and test their extensions. This exposes the
"AllowLegacyMV2Extensions" feature in chrome://flags to let developers
flip it for testing and development purposes.

*This flag is temporary* and will be removed when support for the
enterprise policy is removed.

Bug: 367395349
Change-Id: I2a9d64048011161ba9c79e9a57f1320e1fe783e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5874340
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1381987}