[Autofill Assistant] Add new web-action to get attribute

This CL adds a new web-action to get the attribute of an element.
This is required for extending the |GetElementStatusAction| to
also handle the text content of an element.

The attributes can be nested.

Bug: b/169924567
Change-Id: Id5ff86d907d34d8cbf8e56aa8dfd377fe8f934bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463233
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Luca Hunkeler <hluca@google.com>
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#816111}