zebra, lib, vtysh: Add CLI cmd to set/unset SRv6 encap source address

zebra, lib, vtysh: Add CLI cmd to set/unset SRv6 encap source address

- Add a new node `SRV6_ENCAP_NODE` to the CLI graph. This node allows
users to configure encapsulation parameters for SRv6, including the
source address of the outer encapsulating IPv6 header.

- Install a new CLI command `source-address` under the
`SRV6_ENCAP_NODE` node. This command is used to configure the source
address of the outer encapsulating IPv6 header.

- Install a new CLI command `no source-address` under the
`SRV6_ENCAP_NODE` node. This command is used to unset the
source address of the outer encapsulating IPv6 header and restore the
default source address.

Examples:

```
router# segment-routing
router(sr)# srv6
router(srv6)# encapsulation
router(srv6-encap)# source-address fc00:0:1::1
```

```
router# segment-routing
router(sr)# srv6
router(srv6)# encapsulation
router(srv6-encap)# no source-address
```

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