iwa: Extract `IsolatedWebAppResponseReader` class

This class encapsulates the Isolated-Web-App-specific logic for reading
responses from Signed Web Bundles. The `IsolatedWebAppReaderRegistry`,
which, confusingly, stored `SignedWebBundleReader`s prior to this CL,
now stores `IsolatedWebAppResponseReader`s.

All of this is in preparation for extracting the validation and
verification logic for Isolated Web Apps from the
`IsolatedWebAppReaderRegistry`, so that the validity of an Isolated Web
App can also be checked without having to go through the registry.

In this intermediary commit, `IsolatedWebAppReaderRegistry::Cache` now
stores an `absl::variant`, which will be simplified again in the next
CL.

Bug: 1366309
Change-Id: I1649a9dd3a49b3f016e99a93e6c4597c622b5199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4173652
Commit-Queue: Christian Flach <cmfcmf@chromium.org>
Reviewed-by: Sonja Laurila <laurila@google.com>
Reviewed-by: Oleksandr Peletskyi <peletskyi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1095617}