feat: Add MegaETH Testnet as a default test network

[wantedsystem]

Description

Add MegaETH Testnet to default testnet networks

Related issues

Fixes:

Manual testing steps

• Click on show test networks
• You will see Sepolia / Linea Sepolia and MegaETH Testnet
• Click on MegaETH Testnet
• Send a transaction
• Check the transaction result in the explorer

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues
• I've included manual testing steps
• I've included screenshots/recordings if applicable
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.
• I’ve properly set the pull request status:
  • [X ] In case it's not yet "ready for review", I've set it to "draft".
  • [] In case it's "ready for review", I've changed it from "draft" to "non-draft".

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

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[wantedsystem]

I have read the CLA Document and I hereby sign the CLA