Add BGP template neighbor feature (accept BGP connection from range/prefix)

[nomaster]

I would like templates for BGP neighbor configuration. OpenBGPD does this as follows:

Instead of the neighbor's IP address, an address/netmask pair may be given:
neighbor 10.0.0.0/8
In this case, the neighbor specification becomes a template, and if a neighbor connects from an IP address within the given network, the template is cloned, inheriting everything from the template but the remote address, which is replaced by the connecting neighbor's address. With a template specification it is valid to omit remote-as; bgpd(8) will then accept any AS the neighbor presents in the OPEN message.

[nomaster]

The use case I'm thinking of is the usage of BIO as a route reflector, in which it redistributes announcements of host routes by short-lived systems (such as virtual machines and containers) to the core routers.