Enable Direct Sockets in Chrome Apps behind a dedicated feature flag

This flag is off-by-default; it will be flipped in a separate CL once
all necessary launch approvals are through.

This CL also adds a few tests to validate that bindings are exposed
correctly depending on the presence of the "sockets" key in the
manifest.

Bug: b/329445684

Change-Id: I5beec899ea1fb0c41c243e6fee3b25c63c0f8fc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5300256


Commit-Queue: Andrew Rayskiy <greengrape@google.com>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279234}