iwa: Extract Update Manifest parsing logic from policy handling

This CL extracts the update manifest downloading and parsing logic into
a standalone class, `IsolatedWebAppUpdateManifestFetcher`. This is in
preparation for re-using the update manifest processing for implementing
updating of IWAs.

This CL also introduces a new `UpdateManifest` class, which acts as a
thin wrapper around the data contained in an update manifest.

The parsing logic is now also aligned more closely to the explainer:
- Additional fields are ignored.
- Invalid URLs or versions are ignored, instead of failing parsing.

Update Manifest Explainer: https://github.com/WICG/isolated-web-apps/blob/main/Updates.md

Bug: 1444407
Change-Id: I9e5f088eaab68ef544144e2ce29d1b264e5169f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4521123


Commit-Queue: Christian Flach <cmfcmf@chromium.org>
Reviewed-by: Oleksandr Peletskyi <peletskyi@chromium.org>
Reviewed-by: Sonja Laurila <laurila@google.com>
Reviewed-by: Rainhard Findling <rainhard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149098}