Add distribution_version Signal to Security Reports

The current os_version value on Linux represents the kernel version.
Some dependent systems also need to look at the distribution version
whenever it exists on Linux (via /etc/os-release). Adding that second
version on the side to paint a better picture of the current device's OS
versions.

Bug: b:435197254
Change-Id: Ia4ab06cd5ad2811f6f2c3a04d522e07d76ed8f92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6802266
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Sébastien Lalancette <seblalancette@chromium.org>
Reviewed-by: Hamda Mare <hmare@google.com>
Reviewed-by: Yusuf Sengul <yusufsn@google.com>
Cr-Commit-Position: refs/heads/main@{#1494411}