Add Mantle Chain to Dune

Mantle chain will be on mainnet in July 2023. Mantle has been built with a modular architecture that uses an optimistic rollup with an innovative data availability solution with EigenLayer. By adding Mantle to Dune, it allows Dune community to track overall L2 chains performance and ecosystem growth and trend. More details https://docs.mantle.xyz/network/introducing-mantle/a-gentle-introduction