feat: mtu-1.3: large ip packet transmission (openconfig#2181)

* feat: mtu-1.3: large ip packet transmission

* refactor: set otg l1 speed based on testbed

feature/mtu/largeippacket/otg_tests/large_ip_packet_transmission/README.md

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+# MTU-1.3: Large IP Packet Transmission
+
+## Summary
+
+Test that ports with sufficiently high MTU do not fragment any packets when flows of various size 
+IPv4 and IPv6 packet sizes are sent over them.
+
+## Procedure
+
+* Configure DUT with routed input and output interfaces with an Ethernet MTU of 9216.
+  * Test should be executed with two different interface/connectivity profiles:
+    1) Standalone -- one input and one output port
+    2) Bundle with four input members and four output members
+* Run traffic flows of the following size over IPv4 and IPv6 between ATE ports. 
+  * 1500 Bytes
+  * 2000 Bytes
+  * 4000 Bytes
+  * 9202 Bytes
+* Assert ATE reports packets sent and received count are the same, indicating no fragmentation, and 
+  successful transit.
+
+## Config Parameter coverage
+
+* /interfaces/interface[name=*]/config/mtu:
+* /interfaces/interface[name=*]/subinterfaces/subinterface[index=*]/ipv4/config/mtu:
+* /interfaces/interface[name=*]/subinterfaces/subinterface[index=*]/ipv6/config/mtu:
+
+## Telemetry Parameter coverage
+
+No configuration coverage, validates success by checking flow statistics between ATE ports.
+
+## Protocol/RPC Parameter coverage
+
+N/A
+
+## Minimum DUT platform requirement
+
+N/A
+