chores: added assetchain-testnet in chains folder

[KodeSage]

Added A New Chain- Asset Chain

What is Asset Chain?

Asset Chain is the native blockchain for Xend Finance Real-World Asset OAE (Onchain Asset Environment); with its consensus based on Fantom's Lachesis consensus.
Learnmore

Tasks Performed

Created a new file assetchain-testnet.ts in the packages/thirdweb/src/chains/chain-definitions folder and updated it with the chain contents and requirements.

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PR-Codex overview

The focus of this PR is to add a new asset chain definition for the AssetChain Testnet.

Detailed summary

• Added a new asset chain definition for AssetChain Testnet
• Defined chain with ID 42421, name "AssetChain Testnet", native currency "Real World Asset" (RWA)
• Included block explorer details
• Set testnet flag to true

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Latest commit: 90b10aa

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[jnsdls]

thanks!