Support for configuring IPv4 routes to be sent/withdrawn using Traditional REACH/UN_REACH NLRI.

[apratimmukherjee]

Current model does not provide the control to be able specify that BGP IPv4 routes sent by BGPv4 peer should be sent using Traditional NLRI so implementations are currently expected to normally always send IPv4 routes in MP_REACH and withdraw routes using MP_UNREACH.
This PR allows user to optionally allow IPv4 routes only for IPv4 BGP peers to be configured to be advertised or withdrawn using Traditional REACH/UN_REACH methods .

Redocli view of proposed model
devices[i].bgp.ipv4_interfaces[j].peers[k].traditional_nlri_for_ipv4_routes

Note: This is applicable only for BGPv4 peers and not for BGPv6 peers ( which can send IPv4 routes only using MP_REACH NLRI )

Design note:
Another way to do this could have been to expose this control per route per peer instead of per BGPv4 peer.
Did not choose to go down this path since

1. Does not seem to add much extra value. Normally one specific BGPv4 peer will either send all IPv4 routes as Traditional or as MP_REACH,
2. Since current model is created using shared class/object for BGP v4 routes , adding a new attribute there would mean it would also show up for BGPv6 peer , which is not desired. Adding a new class ( with duplication of remaining attributes ) would result in class name change and possible auto generated enum names within that object ( for snappi/gosnappi) to also change resulting in backward compatibility breakage so this option is not chosen. [ Also makes it difficult for maintenance in terms of any modifications of attributes/documentation ]

Example usage:

//Config creation 
{
	...
	config := gosnappi.NewConfig()

    // add ports / devices/ interfaces as usual 
    ....
    
    // Only showing snippet of BGPv4 peer configuration relevant to this PR . 
     d1Bgp := d1.Bgp().
                SetRouterId("1.1.1.2")

    d1BgpIpv4Interface1 := d1Bgp.
                Ipv4Interfaces().Add().
                SetIpv4Name("p1d1ipv4")

    d1BgpIpv4Interface1Peer1 := d1BgpIpv4Interface1.
                Peers().
                Add().
                SetAsNumber(2222).
                SetAsType(gosnappi.BgpV4PeerAsType.IBGP).
                SetPeerAddress("1.1.1.1").
                SetName("p1d1bgpv4").
                SetTraditionalNlriForIpv4Routes(true)  // This is the newly introduced attribute per BGPv4 Peer.
                 
   // This route will now be transmitted or withdrawn using Traditional NLRI 
    d1BgpIpv4Interface1Peer1V4Route1 := d1BgpIpv4Interface1Peer1.
                V4Routes().
                Add().
                SetName("p1d1peer1rrv4")

    d1BgpIpv4Interface1Peer1V4Route1.Addresses().Add().
                SetAddress("10.10.10.1").
                SetPrefix(32).
                SetCount(4).
                SetStep(1)
      ....

[SuryyaKrJana]

looks fine.