lib: Add new enum for seg6local flavor operations

lib: Add new enum for seg6local flavor operations

The RFC 8986 defines the SRv6 Network Programming concept and specifies
the base set of SRv6 behaviors that enables the creation of
interoperable overlays with underlay optimization. In addition, the RFC
8986 introduces the concept of "flavors", additional operations that can
modify or extend the existing SRv6 behaviors.

This commit adds a new enum type `seg6local_flavor_op` to represent the
SRv6 flavors operations. Currently we define the following flavor
operations:
- PSP (defined in RFC 8986 section #4.16.1)
- USP (defined in RFC 8986 section #4.16.2)
- USD (defined in RFC 8986 section #4.16.3)
- NEXT-C-SID (defined in draft-ietf-spring-srv6-srh-compression-03 #4.1)

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>