esim/http_json_api: extend JSON API with server functionality
At the moment http_json_api only supports the client role. Let's also add support for the server role.
This patch refactors the existing client code. This in particular means that the following preperations have to be made:
- To use the existing JsonHttpApiFunction definitions in the client and server the scheme has to be symetric. It already is for the most part, but it treads the header field differently. So let's just treat the header field like any other mandatory field and add it input_params. (this does not affect the es9p.py code since in ES9+ the requests have no header messages, see also SGP.22, section 6.5.1.1)
- The JsonHttpApiFunction class currently also has the code to perform the client requests. Let's seperate that code in a JsonHttpApiClient class to which we pass an JsonHttpApiFunction object.
- The code that does the encoding and decoding in the client role has lots of conditions the treat the header differently. Let's do the decisions about the header in the JsonHttpApiClient. The encoder and decoder function should do the generic encoding and decoding only. (however, some generic header specific conditions will remain).
The code for the server role logically mirrors the code for the client role. We add a JsonHttpApiServer class that can be used to create API endpoints. The API user has to pass in a call_handler through which the application logic is defined. Above that we add an Es2pApiServer class in es2p. In this class we implement the logic that runs the HTTP server and receives the requests. The Es2pApiServer supports all ES2+ functions defined by GSMA SGP.22. The user may use the provided Es2pApiServerHandler base class to define the application logic for each ES2+ function.
