shortcuts: Add shortcut accelerator provider

Add shortcut accelerator provider for customized shortcut actions
instead of hardcoding a set of fixed shortcuts in the frontend.

Followup cl about how mojo is used in frontend:
https://crrev.com/c/5302340

Test: ash_webui_unittests
Bug: b/216049298
Change-Id: I332651de94967cf0607b7c9a8c2c415995fb7e96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5298014


Commit-Queue: Yuhan Yang <yyhyyh@google.com>
Reviewed-by: Chris Bookholt <bookholt@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: David Padlipsky <dpad@google.com>
Cr-Commit-Position: refs/heads/main@{#1281971}