fpm: Add func to encode SRv6 SIDs in protobuf

fpm: Add func to encode SRv6 SIDs in protobuf

Add a bunch of helper functions to support encoding of SRv6 SIDs in
protobuf.

Specifically,
* `fpm_srv6_sid_format_create`: encode SID format information into a
protobuf `SRv6SIDFormat` structure
* `fpm_srv6_local_sid_*_behavior_create` functions: encode an SRv6 SID
behavior and associated attributes into a protobuf
`Fpm__SRv6LocalSID__*` structure

This is a preliminary commit to support sending SRv6 Local SIDs and VPN
SIDs via protobuf.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>