IP options in IPv4 header

[indraniBh]

ReDocly link:
ReDoc Interactive Demo (redocly.github.io)

( New object at : set_config/flows/packet/ipv4/options )

IPv4 RFC: RFC791
Router Alert Option RFC: RFC2113
IPv4 Options parameters

Objective: IPv4 options are optional extensions for the IPv4 header that can be utilised to provide additional information about the IPv4 datagram. It is encoded as a series of type, length and value attributes. The IP header length MUST be increased to accommodate the extra bytes needed to encode the IP options.
Currently IP options supported are:

1. router_alert Option allows devices to intercept packets not addressed to them directly as defined in RFC2113.
2. custom option is provided for to be able to configure user defined IP options as needed.
   IPv4 Options is an array option objects.
   To support RSVP control packet, router alert option is required in IP header.

Sample IPv4 Options (Router Alert and Custom):

        srcIP := "10.10.0.1"
	dstIP := "10.10.0.2"

	config := gosnappi.NewConfig()
	// add ports
	p1 := config.Ports().Add().SetName("p1").SetLocation("eth1")
	p2 := config.Ports().Add().SetName("p2").SetLocation("eth2")

	f1 := config.Flows().Items()[0]
	f1.SetName("f1:p1->p2").
		TxRx().Port().SetTxName(p1.Name()).SetRxName(p2.Name())
	f1.Duration().FixedPackets().SetPackets(100)
	f1.Rate().SetPps(10)

	f1.Packet().Add().Ethernet()
	ip := f1.Packet().Add().Ipv4()

	ip.Src().SetChoice("value").SetValue(srcIP)
	ip.Dst().SetChoice("value").SetValue(dstIP)

	// Sample of router_alert option:
	ip.Options().Add().SetChoice("router_alert")

	// Sample of user defined custom TLV options (Stream ID)
	ipOptionCustom := ip.Options().Add().SetChoice("custom")
	ipOptionCustom.Custom().Type().CopiedFlag().SetChoice("value").SetValue(0)
	ipOptionCustom.Custom().Type().OptionClass().SetChoice("value").SetValue(0)
	ipOptionCustom.Custom().Type().OptionNumber().SetChoice("value").SetValue(8)

	ipOptionCustom.Custom().Length().SetChoice("value").SetValue(4)
	ipOptionCustom.Custom().SetValue("0088")

[rudranil-das]

Please update example by actually adding item in ip.Options() for usage in each of the choices

[rudranil-das]

rest LGTM

[apratimmukherjee]

Minor documentation comments. Rest looks good to me.

[apratimmukherjee]

Please update the example to reflect the changes for adding custom option. Also need to actually add option to packet template I think as @rudranil-das had mentioned to make it a valid snippet.

[indraniBh]

Updated code snippet. @rudranil-das @apratimmukherjee