Make add_key Free On-Chain for Eligible Keys

[p5150j]

Description

Modify on-chain logic to make the add_key operation free for eligible keys, reducing friction in the migration process.
In the first phase of ETH key migrations we need to allow users to add one ETH compatible key as a free transaction.

Tasks

• Implement fee waiver logic in on-chain code
• Create validation mechanisms to prevent abuse
• Test edge cases for eligibility determination

Acceptance Criteria

• add_key operation is free for all eligible keys (a tiny capacity amount)
• Non-eligible keys still require appropriate fees
• Abuse prevention mechanisms are in place
• Transaction processing correctly handles fee-free operations (a tiny capacity amount)
• All edge cases tested and working correctly
• Documentation updated to explain eligibility criteria

Engineering Notes

• eligibility: Any MsaId that has only one key before a certain block_number or msa_id (this block number or msa id would be the cutover window which all new Accounts are created with a ETH compatible key after that). This cutover value or criteria should be configurable