Add hidden mint underlying token CLI command

[mkysel]

Add a hidden settlement underlying-mint CLI command and implement settlementChainAdmin.MintMockUSDC with amount bounds ≤ 10,000,000,000 for minting mock underlying fee tokens

• Add a hidden underlying-mint subcommand under settlement in settlement.go that mints mock underlying fee tokens with --to, --amount, and --raw flags, delegating to settleUnderlyingMintHandler.
• Introduce MintMockUSDC(ctx, addr, amount) error on ISettlementChainAdmin and implement it in settlement_chain_admin.go using a MockUnderlyingFeeToken binding to perform on-chain mint with validation and event logging.
• Generate Go bindings for MockUnderlyingFeeToken in MockUnderlyingFeeToken.go and update ABI generation in abi.
• Extend config to carry the mock underlying fee token address via JSON and flags/env in blockchain.go, options.go, and validation.go; update test helpers in config.go.
• Add currency.MicroDollar.ToBigInt() in currency.go.

📍Where to Start

Start with the CLI entrypoint and handler in settlement.go, then follow the call into settlementChainAdmin.MintMockUSDC in settlement_chain_admin.go.

Changes since #1129 opened

• Modified settlementChainAdmin.MintMockUSDC method to parse MockUnderlyingFeeToken Transfer events instead of Migrated events [165963e]

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Macroscope summarized 165963e.

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