FEP-ae97: Client-side activity signing

https://codeberg.org/silverpill/feps/src/branch/main/ae97/fep-ae97.md

I've been experimenting with client-side signing for quite some time and decided to write a document describing a complete solution. It builds on my past proposals (FEP-8b32 and FEP-c390), and if widely implemented, will allow users to send activities from anywhere, without tying them to a particular server.

I'm still not sure if client-side signing makes sense for Mitra, but some other projects like Vocata might support this.