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.

---

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

[mergify]

Cherry-pick of 7b1c0c2 has failed:

On branch mergify/bp/stable/9.0/pr-16354
Your branch is up to date with 'origin/stable/9.0'.

You are currently cherry-picking commit 7b1c0c23fc.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

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

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally