bgpd: Fix SRv6 SID index limit

bgpd: Fix SRv6 SID index limit

Previously BGP supported up to 255 SIDs.

The PR https://github.com/FRRouting/frr/pull/11981 extended the
transposition computation algorithm in BGP to support more SIDs (up to
1048575 SIDs).

However the BGP VTY command for allocating an SRv6 per-VRF SID
(`sid vpn per-vrf export`) is still limited to 255 SIDs.

This commit extends the SID index in `sid vpn per-vrf export` VTY
command to support up to 1048575 SIDs.

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