Add builtin network configuration

[jframe]

Description

Add builtin network configurations for Linea mainnet and Sepolia testnet to Maru. This would allow users to easily connect to these networks without manually configuring the genesis file. Users would be able to start Maru with a simple --network=linea-mainnet or --network=linea-sepolia flag instead of providing custom configuration files.

Motivation

• Improved user experience: Reduces friction for users wanting to test Maru on public networks
• Reduced configuration errors: Pre-validated configs eliminate common setup mistakes
• Faster onboarding: New users can quickly spin up a node without hunting for correct network parameters

Tasks

• Create CLI flag/arguments (e.g., --network=linea-mainnet|linea-sepolia)
• Add network configuration for Linea Sepolia and Linea Mainnet networks
• Add network configuration loading logic

Acceptance criteria

• User can start Maru with --network=linea-mainnet and connect to Linea mainnet
• User can start Maru with --network=linea-sepolia and connect to Linea Sepolia testnet

Risks

Remember to

• Add the documentation label in case there is an impact on the documentation
• Add priority and team labels
• Add Task for updating the Runbook or adding/updating existing metrics and alerts