[Feat]: Add IntentMandate support for human-not-present flows with cryptographic session keys and programmable spending rules

[jaytoday]

Is your feature request related to a problem? Please describe.

Current AP2 only supports human-present payment flows where users must manually confirm each transaction.

Describe the solution you'd like

Improved support for human-not-present flows, including support for programmable spending rules so that the user can provide parameters for autonomous agent spending.

Describe alternatives you've considered

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[jaytoday]

PR to add this feature: #77