Include exact host matching for actor allowlist

The existing allowlist parameter will match subdomains if the
superdomain is in the allowlist. We introduce another allowlist
parameter which only matches on the exact host for cases where we do not
want to allow subdomains.

For example, `allowlist=example.com` would match both example.com and
sub.example.com, but `allowlist_exact=example.com` would only match
example.com.

Bug: 407572652
Change-Id: Ia0c8e9bc6e1b42a05386eff5c52a0fb01d52c659
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6418062
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440514}