Add 'bgp.link_bandwidth' knob to the bgp.session plugin

[jbemmel]

Enables weighted ECMP based on link capacity

Inspired by https://bgplabs.net/lb/2-dmz-bw/

[ipspace]

Love the idea. However, I think this functionality should be part of the bgp.policy plugin:

• It modifies existing routing updates, and we're doing that in bgp.policy for weight, LocPref, MED, and communities
• On some platforms you have to set Link Bandwidth with a route map, and the bgp.policy plugin already has the framework to deal with platforms that can set attribute X either directly or with a route map.

Also, in theory you could set Link Bandwidth on incoming or outgoing updates, so we might need bgp.bandwidth.in and bgp.bandwidth.out (with a number instead of a dictionary meaning in).

I could write the framework Python code in bgp.policy plugin, and then you'd just add the SR OS template.

[jbemmel]

Sure, we can put it there. There are indeed additional knobs to send or accept the extended community to/from ebgp peers.

Let me know when you're ready for the template and I'll add it