net/frr: move peer group policy into address families

[Monviech]

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Describe the problem

Peer groups gained an explicit address family selector, but only the activation command was rendered inside the selected address-family block. Policy-related settings such as prefix-lists, route-maps, next-hop-self and default-originate were still emitted in the global peer group section.

This caused IPv6-only peer groups to be activated under IPv6 while their policy configuration was generated outside of the IPv6 address-family context, where it would not apply as expected.

Move these AF-scoped peer group settings into the address-family loop and keep only session-level settings in the global peer group block. Add small helpers for prefix-list and route-map attachment rendering so the same logic can be shared between normal neighbors and peer groups.

Fixes: #5506

[Monviech]

Asked for feedback here:
#5506 (comment)

But I also tested the change myself and it looks good.