feat: add AWS KMS signer

[colinlyguo]

Purpose or design rationale of this PR

This PR introduces AWS KMS signer support for secure transaction execution. To use this feature, you'll need cast send --aws along with proper AWS KMS access permissions for executing admin transactions, such as acceptOwnership in Ownable2Step.sol and restricted functions in VerifyingPaymaster.sol that require onlyOwner privileges.

Security Note: All AWS KMS operations will be automatically logged in AWS CloudTrail for auditing purposes.

Usage example:

export AWS_KMS_KEY_ID="arn:aws:kms:REGION:ACCOUNT:key/KEY-ID"
cast send --aws \
  --rpc-url "$RPC_END_POINT" \
  --gas-limit 100000 \
  --gas-price 20000000000 \
  "$PAYMASTER_ADDRESS" \
  "acceptOwnership()"

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• feat: A new feature

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change