iwa: add list of update channels to IsolatedWebAppUpdateServerMixin

This cl adds the ability to generate update manifests with lists of
update  channels via IsolatedWebAppUpdateServerMixin.

Change-Id: I6d1de1be987275d41aaf2c80d508395aa9b0397d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5920796
Reviewed-by: Andrew Rayskiy <greengrape@google.com>
Commit-Queue: Magdalena Skarbińska <mskarbinska@google.com>
Reviewed-by: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#1368158}