system-extensions: Add command line switch for installing

Bug: 1192426
Change-Id: Ib8e1be66dc135d71cf8ed34742071ffc6020f133
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3120967


Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918729}