HC2VPP-258: add support for MPLS label push

HC2VPP-258: add support for MPLS label push

Translates impose-and-forward operation of
/hc2vpp-ietf-routing:routing/hc2vpp-ietf-mpl:mpls/
hc2vpp-ietf-mpls-static:static-lsps/static-lsp

to ip_add_del_route call.

For single outgoing-label, use simple-path out-segment.
For multiple outgoing labels, use path-list out-segment
with single path (multiple path support is not implemented yet).

See postman collection for examples.

TODOs:

- only IPv4 prefixes and next-hops are supported (HC2VPP-264).

Change-Id: I318e722edbc1f7ffd3eff2f308fc73c2062283c0
Signed-off-by: Marek Gradzki <mgradzki@cisco.com>