Add SendAsync to TxMgr

[mslipper]

Adds a SendAsync method to TxMgr. I'd like to use this for op-deployer, which needs to send multiple transactions in parallel but with predictable nonces. SendAsync returns a channel that resolves with the result of each send, but synchronously increases the nonce and prepares the first send prior to returning.

[semgrep-app]

Semgrep found 8 sol-style-return-arg-fmt findings:

• packages/contracts-bedrock/src/libraries/Constants.sol
  • L42 - Triage
• packages/contracts-bedrock/src/L1/SystemConfig.sol
  • L436 - Triage
• packages/contracts-bedrock/src/L1/OptimismPortal2.sol
  • L275 - Triage
• packages/contracts-bedrock/src/L1/OptimismPortal.sol
  • L225 - Triage
• packages/contracts-bedrock/scripts/interfaces/ISystemConfigV0.sol
  • L17 - Triage
• packages/contracts-bedrock/scripts/DeploySuperchain.s.sol
  • L115 - Triage
  • L196 - Triage
  • L262 - Triage

Named return arguments to functions must be appended with an underscore (_)

_{Ignore this finding from sol-style-return-arg-fmt.}