Allow setting BGP max-paths for ECMP

[nickzxcv]

Is your feature request related to a problem? Please describe.

In the network design using ECMP with BGP routing to some F5s, there may be multiple routers in front of each F5. The F5 should be able to load balance response traffic back to the Internet via the multiple routers using ECMP as well as ECMP in the edge router for traffic coming to the F5.

Describe the solution you'd like

In the RoutingBGP declaration or wherever appropriate, to have an option for setting max-paths such as

"exampleRoutingBGP": {
  "class": "RoutingBGP",
  "maxPaths": 2
}

Describe alternatives you've considered

Configuring max-paths using tmsh after DO is complete, according to the tmsh options described in
https://clouddocs.f5.com/cli/tmsh-reference/latest/modules/net/net_routing_profile_bgp.html

Additional context

Based on DO schema 1.42.0