Implement bgp.advertise attribute to advertise networks in BGP without adding a static route

[ipspace]

The bgp.originate attribute allows you to advertise BGP prefixes supported by static discard routes. Sometimes we'd like to advertise BGP prefixes that were learned via some other protocol without doing protocol redistribution. Most BGP implementations can do that, either with network statements or routing policies. There's no reason we wouldn't have something similar in netlab.