Implement Third-Party Block Handling

[Naccomy]

Hey, first of all thank you for your work on this repository! I’d like to contribute to adding the third party block feature!
Do you have any preferences on how this feature should be implemented, I thought about either:

• Mimic the implementation here with ThirdPartyRequest and ThirdPartyBlock.
• Adding a new function to the Biscuit structure to append a third-party block.
• Other ideas?

Please let me know how I can help!