bgpd: add bgp ipv6-auto-ra command (backport #16354)

[mergify]

Introduce a command to completely disable sending IPv6 router advertisement messages on an interface. Before, there were cases where it could be enabled by other daemons, namely bgpd and vrrpd. Particularly, it happens when BGP extended-nexthop capability is used: bgpd tells zebra to enable RA in this case, though it wasn't intended.

Fixes #7738.

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This is an automatic backport of pull request #16354 done by [Mergify](https://mergify.com).

[mergify]

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On branch mergify/bp/stable/8.5/pr-16354
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Changes to be committed:
	modified:   bgpd/bgp_zebra.c
	modified:   doc/user/bgp.rst
	modified:   doc/user/ipv6.rst

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   bgpd/bgp_nht.c
	both modified:   bgpd/bgp_vty.c
	both modified:   bgpd/bgpd.c
	both modified:   bgpd/bgpd.h

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