Define interface and behavior for the ICorrespondenceClient

[martinothamar]

Define an interface called ICorrespondenceClient. Consider different approaches for designing the public types to make it simple to construct Correspondence messages such as builder pattern and Value Objects/domain models

Acceptance criteria

• There is a method using the /upload endpoint, which sends all the data as multipart formdata
• There is a method for checking the status of a correspondence
• It is clear from the XML docs the responsibility of producing correspondences (what does it mean to successfully post a correspondence, when do users have to handle errors, how etc)