Changes

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 tread 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 tread 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.

Related: SYS#7825
Change-Id: I277aa90fddb5171c4bf6c3436259aa371d30d092