zebra: CLI to specify format of an SRv6 locator

zebra: CLI to specify format of an SRv6 locator

Add the CLI to choose the SID format of a locator. When the SID format
of a locator is changed, the SIDs allocated from that locator might no
longer be valid (for example, because the new format might involve a
different SID allocation schema). In such a case, it is necessary to
notify all the zclients so that they can withdraw/uninstall the old SIDs
that use the previous format and allocate/install/advertise the new SIDs
based on the new format.

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