chore(server): split context-server to its own file

This is in order to improve maintainability and re-usability by
downstream projects such as LocalAI (see
mudler/LocalAI#5379 for context).

The context server is a struct that can be re-used quite heavily by
other communication protocols. For instance, LocalAI uses the context
server on top of gRPC rather than having a REST API. This change
improves overall re-usability by isolating the REST API to its own file
so the context server can be imported easily.

Signed-off-by: mudler <[email protected]>

Author: mudler