onc: Add MTU field in StaticIPConfig (IPConfig) struct

VPN services will use this field to configure the customized MTU value
on the VPN interface.

Bug: b/320355079
Test: out/Default/chromeos_unittests --gtest_filter="*ONC*"
Change-Id: I8d7c987db95815b37be344c21818fd4a2a9ba6f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5612814


Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Hugo Benichi <hugobenichi@google.com>
Commit-Queue: Jie Jiang <jiejiang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313184}