Add and implement NMPlatformRoutingRule types and let the platform cache
handle rules.

Rules are special in two ways:

- they don't have an ifindex. That makes them different from all other
  currently existing NMPlatform* types, which have an "ifindex" field and
  "implement" NMPlatformObjWithIfindex.

- they have an address family, but contrary to addresses and routes, there
  is only one NMPlatformRoutingRule object to handle both address
  families.

Both of these points require some special considerations.

Kernel treats routing-rules quite similar to routes. That is, kernel
allows to add different rules/routes, as long as they differ in certain
fields. These "fields" make up the identity of the rules/routes. But
in practice, it's not defined which fields contribute to the identity
of these objects. That makes using the netlink API very hard. For
example, when kernel gains support for a new attribute which
NetworkManager does not know yet, then users can add two rules/routes
that look the same to NetworkManager. That can easily result in cache
inconsistencies.

Another problem is, that older kernel versions may not yet support all
fields, which NetworkManager (and newer kernels) considers for identity.
The older kernel will not simply reject netlink messages with these unknown
keys, instead it will proceed adding the route/rule without it. That means,
the added route/rule will have a different identity than what NetworkManager
intended to add.