Configurable BGP sessions

[ipspace]

Add bgp.sessions attribute that will control whether to create IPv4 or IPv6 IBGP or EBGP sessions. We'll document this attribute as an advanced attribute that should be avoided, but it's nice to have a way to tweak the BGP neighbor generation process.

For example, this is what a SRv6 module could set up to enable v4-over-SRv6 service.

bgp.sessions:
  ipv4: [ ebgp ]
  ipv6: [ ibgp, ebgp ]

Please note that this will NOT affect the configuration templates.