Implement FEP-8b32: Object Integrity Proofs

With this, objects and activities can be signed so that the auther is verifiable independent of the storage location.

I am almost finished with the object integrity implementations (in the context of VCs and capabilities). We would need to use a different canonicalization / crypto suite though. There is a library available so this should be very easy to add.

The question that remains: Where to use this and how to design in the UI. The other impelemtations might be a good starting point for investigation.

https://codeberg.org/fediverse/fep/src/branch/main/fep/8b32/fep-8b32.md
https://socialhub.activitypub.rocks/t/fep-8b32-object-integrity-proofs/2725

Read the FEP-8b32 specification and the linked SocialHub discussion first, then inspect the other object-integrity implementations mentioned in the issue. Determine where signing should be used and how it should be represented in the UI; done means the FEP is implemented for objects and activities with a clear integration design.

Written by the indexing model from the issue text.