ONC: Move ONC code into chromeos/components/onc

Move ONC related files from chromeos/network/onc into
chromeos/components/onc. No behavior change is intended.

This CL moves the files that do not depend on Ash (i.e. on Shill, etc)
into the component, so they can be reused. This is required
to implement the OpenNetworkConfiguration policy in Lacros.

onc_utils.cc and onc_utils_unittest.cc are splitted. The methods
and tests related to the moved files are also moved and the rest
is related to the remaining files in network/onc.

The files were not moved into components/onc because components/onc is
compiled for non-ChromeOS platforms too as it is used by the
networking.onc / networkingPrivate extension APIs.

components/onc contains ONC constant definitions usable on any platform.
chromeos/components/onc contains ONC handling code that is intended to
be shared between ash-chrome and lacros-chrome on Chrome OS.

Bug: b/197082588
Test: Compilation, *Onc*, *ONC* unit tests
Change-Id: I27e9257a819cc849fbfb3146b9e31afc35ea65cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3285465
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/main@{#950516}