feat: Add Across Gas Station support

[pedronfigueiredo]

Summary

• Add a shared Gas Station helper and keep Relay using it via the existing relay helper path.
• Let Across quote source-network fees in the source token when native balance is insufficient and Gas Station is eligible.
• Re-quote max-amount Across flows after reserving source token for gas-fee-token payment.
• Pass gasFeeToken when submitting Across source transactions that use source-token gas payment.

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Note

Medium Risk
Changes how Across quotes compute and represent source-network fees when native balance is low, and alters submission options by adding gasFeeToken, which could affect transaction affordability and execution on EIP-7702 chains.

Overview
Adds a shared gas-station helper (and has Relay re-export it) to price gas in the source token via TransactionController:getGasFeeTokens when the chain is eligible.

Updates Across quoting to (1) switch fees.sourceNetwork to gas-fee-token pricing when native balance is insufficient and gas station can price the source token, and (2) for max-amount requests, perform a second quote after subtracting the reserved gas amount from the source amount with safe fallbacks to the original quote.

Updates Across submission to pass gasFeeToken (source token address) to both addTransaction and addTransactionBatch when fees.isSourceGasFeeToken is set, with expanded tests covering the new behaviors.

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