[ios][build] Build application extensions in a separate toolchain

Add new toolchains to build application extension (widgets, ...)
and configure the different extensions to use it. This allow to
disable PA for the extensions and will eventually allow passing
`-fapplication-extension` to catch at compile time usage of API
not available in application extensions.

Bug: 1055812
Fixed: 1449010
Change-Id: Iacc726e4bc178407094772a2dada3fe8bf08a82a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4580730


Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Ernesto Izquierdo Clua <eic@google.com>
Cr-Commit-Position: refs/heads/main@{#1152640}