In this commit, we introduce the ability to specify flags for an SRv6
locator. Flags can be used to specify the properties of the locator.

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

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>

In this commit, we extend the ZAPI to support encoding and decoding the
locator flags contained in the messages exchanged between zebra and the
routing daemons.

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

In this commit, we introduce a new enumeration to encode the SRv6
Endpoint Behaviors codepoints defined in the IANA SRv6 Endpoint
Behaviors Registry
(https://www.iana.org/assignments/segment-routing/segment-routing.xhtml).

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

Currently bgpd uses the opaque codepoint (0xFFFF) in the BGP
advertisement. In this commit, we update bgpd to use the SRv6 codepoints
defined in the IANA SRv6 Endpoint Behaviors Registry
(https://www.iana.org/assignments/segment-routing/segment-routing.xhtml)

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

In this commit, we extend to print the `SRV6_LOCATOR_USID` flag.
The output appears as follows:

```
{
  "locators":[
    {
      "name":"loc1",
      "prefix":"fc00:0:1::/48",
      "blockBitsLength":32,
      "nodeBitsLength":16,
      "functionBitsLength":16,
      "argumentBitsLength":0,
      "uSID":true,
      "statusUp":true,
      "chunks":[
        {
          "prefix":"fc00:0:1::/48",
          "proto":"bgp"
        }
      ]
    }
  ]
}
```

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

In this commit, we add two helper functions
`zebra_notify_srv6_locator_add` and `zebra_notify_srv6_locator_delete`.
These functions are used to notify locator additions/deletions to
zclients.

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

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>

This test ensures that the command `behavior usid` works properly.

When the `behavior usid` command is set, a flag is added to the locator
to indicate that the locator is a uSID locator. This test verifies that
the locator works correctly when you set / unset the `behavior usid`
command.

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

Add the documentation for the `behavior usid` command to zebra.

When the `behavior usid` command is set, a flag is added to the locator
to indicate that the locator is a uSID locator. When a locator is
specified as a uSID locator, the bgpd will install SRv6 behaviors with
the uSID in the dataplane and use the SRv6 uSID codepoints in the BGP
update message.

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