Design "endcap" architecture

[warriordog]

ActivityPub Sharp is intended to expose multiple tiers of APIs. The lowest-level API is the raw parsers and models available in the Types package, and the mid-level API consists of use cases that will be defined in #117. This story is to create the high-level architecture, which is a pair of "endcap" packages which bring together the most important use cases into a unified entry point.

The first endcap package will be for client usage. Part of it, IActivityPubClient, has already been created. These additional features must be designed as well:

• Per-server client
• Stateful client
• Navigational API (smart entities, basically)

The other endcap is for servers. The purpose here is to provide a single entry point to process an incoming request

• ActivityPub server - processes arbitrary objects
• HTTP server - processes HTTP requests (through an abstraction)

Everything here is a long-term goal, the last stage of the project really. But we need to work out the vision now since everything else will be building to this.