samples: net: cellular_modem: Add additional overlay for nrf7002dk

[ethansisk]

This overlay is for the nrf7002dk. The modem tested was the Telit ME310G1-WW, but should work for any of the supported modems provided that the UART lines are defined and hooked up correctly.

[github-actions]

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[bjarki-andreasen]

For local board modifications, one should use snippets locally. Local board modifications can't go upstream since overlays apply to the base board, so the overlay would be applied, likely incorrectly, to everyone elses local board, which could be configured very differently :)

To run the cellular sample with snippets, you would create the snippet with the overlay you add in this PR (named "nrf7002dk_telit" in this example), then build the sample like

west build -p -b nrf7002dk/nrf5340/cpuapp --snippet nrf7002dk_telit zephyr/samples/net/cellular_modem

With the base board and snippet, you have a "custom" local board you use for any in-tree sample