working l2vlan sub-interface

[wungad]

This PR adds a working functionality when dealing with iana:l2vlan interface types. It requires that parent-interface exists and that it references an existing entry in the ietf-interfaces:intefaces list.

Without the change, interface plugin fails when doing the initial load:

[INF] ietf-interfaces-plugin: Loading initial system data
[ERR] Mandatory node "parent-interface" instance does not exist. (Schema location "/ietf-interfaces:interfaces/interface/ietf-if-extensions:parent-interface".)
[ERR] Validation failed.
[ERR] ietf-interfaces-plugin: sr_apply_changes() error (9): Validation failed
[ERR] srpc: /src/sysrepo-plugin-interfaces/src/interfaces/src/plugin.c:310 interfaces_running_load(ctx, running_session) error (-1)
[ERR] ietf-interfaces-plugin: Error occured while initializing the plugin (-1)
[ERR] sysrepo-plugind: Plugin "libsrplg-ietf-interfaces" initialization failed (User callback failed).
[INF] No datastore changes to apply.

This PR also makes it possible to add/change/remove ipv4/ipv6 routes using the routing plugin.

Note:
The issue doesn't affect setups that do not use iana:l2vlan (e.g. eth0.231) type interfaces and so could not be detected until now.