zebra: Add `behavior usid` command to VTY

zebra: Add `behavior usid` command to VTY

Install a new command `behavior usid` into the `SRV6_LOC_NODE` CLI node.

This command allows the user to set/unset the `SRV6_LOCATOR_USID` flag
for an SRv6 locator. The `SRV6_LOCATOR_USID` flag indicates whether a
locator is a uSID locator or not. When the flag is set, the routing
daemons (e.g., bgpd) will install SRv6 behaviors with the uSID in the
dataplane.

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