IPv6 Support with single stack networks

[majst01]

Actions Required

The ChildPrefixLength property was removed from the Partition Entity. Every TenantSupernetwork must now be configured with ChildPrefixLength accordingly.
This is done during startup in the database migration step and set to the same value as in the `Partition`.

Depends on:

• Full IPv6 Support docs#202
• IPv6 Support metalctl#252
• IPv6 Support mini-lab#175
• IPv6 Support metal-core#127
• Support IPv6 metal-networker#110
• IPv6 Support metal-images#247

replaces #152

TODO:

• validate update request
• store addressfamily in network immutable and validate prefixes on update

[majst01]

With: https://docs.frrouting.org/en/latest/bgp.html#clicmd-neighbor-A.B.C.D-activate

test6# show bgp ipv6 unicast
BGP table version is 4, local router ID is 169.254.255.255, vrf id 0
Default local pref 100, local AS 4210000020
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

    Network          Next Hop            Metric LocPrf Weight Path
 *> ::/0             fe80::a8c1:abff:fea3:c28f(test6)
                                          1024         32768 ?
 *> fd00:a::1/128    ::(test6)                0         32768 ?
 *= fd00:a::2/128    lan1                                   0 4200000012 4210000030 ?
 *>                  lan0                                   0 4200000011 4210000030 ?

[Gerrit91]

This one is only kept for reference and is superseded by #549.