ICRC-25 reference implementation

[jsamol]

This PR adds a reference implementation for the ICRC-25 standard. It includes:

• an example relying party
• an example signer
• a simple target canister implementing the ICRC-21 standard

The implementation in its current state is outdated and is based on the draft version from #32.

[frederikrothenberger]

Thanks a lot for all the work! It still needs some polishing, but I think it is a good start. 🙂

[jsamol]

@frederikrothenberger I updated the PR according to the feedback, I hope I haven't missed anything.