lib: Add `SRV6_LOCATOR_USID` flag to SRv6 locator

lib: Add `SRV6_LOCATOR_USID` flag to SRv6 locator

In this commit, we add support for a new flag called
`SRV6_LOCATOR_USID`. When the `SRV6_LOCATOR_USID` flag is set, the
routing protocols will install SRv6 behaviors with the uSID in the
dataplane.

This flag is used to specify a locator as a uSID locator. When a locator
is specified as a uSID locator, all the SRv6 SIDs allocated from the
locator by the routing protocols (like BGP) are bound to the SRv6 uSID
behaviors and use the SRv6 uSID codepoints in the BGP update message.

We extend the SRv6 locator implementation to add support for a `usid`
flag. When the `usid` flag is set, the bgpd will install SRv6 behaviors
with the uSID in the dataplane and use the proper SRv6 Endpoint Behavior
codepoint in the BGP advertisement.

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