Add network input with env var

[AnieeG]

Motivation

The current way of providing network input to test is through SELECTED_NETWORKS env var. If we need to run tests in more than 2 networks over multiple lanes, the network input in SELECTED_NETWORKS runs the test for all possible combination of lanes between 2 networks among all provided networks. For example with SELECTED_NETWORKS=networkA, NetworkB, NetworkC the test is triggered for all possible combination of these networks - A,B ; B,C; A,C
Often we need to run tests for only specific set of lanes between various networks.

Solution

Introduce another env var CCIP_NETWORK_PAIRS example usage - CCIP_NETWORK_PAIRS="networkA,networkB|networkC,networkD|networkA,networkC"
It's|delimited list of network pairs. It will help us specify lanes on which tests are to be triggered as opposed to all possible combination of lanes