zebra: Alloc/Release SIDs to daemons upon request

zebra: Alloc/Release SIDs to daemons upon request

Previous commits introduced two new ZAPI operations,
`ZEBRA_SRV6_MANAGER_GET_SRV6_SID` and
`ZEBRA_SRV6_MANAGER_RELEASE_SRV6_SID`. These operations allow a daemon
to interact with the SRv6 SID Manager to get and release an SRv6 SID,
respectively.

This commit extends the SID Manager by adding logic to process the
requests `ZEBRA_SRV6_MANAGER_GET_SRV6_SID` and
`ZEBRA_SRV6_MANAGER_RELEASE_SRV6_SID`, and allocate/release SIDs to
requesting daemons.

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