bgp: T8223: Prevent `advertise-all-vni` in multiple BGP VRF instances simultaneously

FRR only allows one BGP instance to hold `advertise-all-vni` at a time
(FRR issue #9405). When a default BGP instance is present it is always
started before named VRF instances, so if a named VRF holds
the flag FRR silently rejects it on every boot (regardless of default
EVPN config), causing the running config to diverge from what is
stored in VyOS.
Enforce the following policy in verify():
- Default BGP instance may always hold `advertise-all-vni`.
- A named VRF may hold it only when no default BGP instance exists.
- Only one BGP instance (default or named VRF) may hold it at a time.
The default BGP verify path additionally scans dependent VRFs so that
adding or modifying the default BGP instance while a named VRF already
holds the flag is caught even when the VRF node is not part of the
current commit.

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