pw_software_update: Add user_manifest bundle arg
pw_software_update: Add user_manifest bundle arg

This change adds a user-defined "user_manifest" option to the GN
template for creating update bundles. The user manifest is treated like
any other target payload for the purpose of bundle creation, but has a
unique API on the GN side because it will be treated differently in the
update client on the firmware side.

Tested: Built downtream with and without user_manifest
Change-Id: Id15d8138e17e4a85808ae04b3c014be5f6e3ea18
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/65360
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Joe Ethier <f5eb8cb2f5310c7fa9df6f16eabccb4436f47999@google.com>
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed