feat: add flexible retryable ticket encoding primitive

[yahgwai]

Context

The existing ParentToChildMessageCreator.getTicketCreationRequest() requires a child chain provider for gas estimation. This makes it unusable for pre-chain-startup scenarios (like ownership transfer during chain deployment) where the child chain isn't running and gas params must be hardcoded.

Scope

New function: encodeRetryableTicket

• Accepts target address, calldata, l2CallValue, refund addresses
• Gas params (gasLimit, maxFeePerGas) provided directly -- no child chain estimation required
• Calculates maxSubmissionCost from parent chain via Inbox.calculateRetryableSubmissionFee() (or accepts it pre-calculated)
• Calculates deposit: maxSubmissionCost + (gasLimit * maxFeePerGas) + l2CallValue
• Handles ETH vs ERC20 native token encoding differences (extra tokenTotalFeeAmount arg)
• Returns { data: encodedCalldata, deposit, maxSubmissionCost }
• Does NOT prepare a transaction request -- just encoding and math

Refactor: getTicketCreationRequest

• Refactor to use encodeRetryableTicket internally after estimating gas params
• No behavior change, just deduplication

New function (optional): encodeSendL2Message

• Takes a signed child-chain transaction
• Prepends the message type byte (0x04 for signed tx)
• Returns encoded calldata for Inbox.sendL2Message
• Simple utility used in the same ownership transfer flow

Out of scope

• Viem support (handled in arbitrum-chain-sdk)
• Schema definitions
• Example scripts

Motivation

This is needed to support a transferOwnership scripting workflow in arbitrum-chain-sdk, where retryable tickets must be constructed for a child chain that hasn't started producing blocks yet. The encoding primitive is also generally useful for any cross-chain operation where gas params are known in advance.