Attach signature from interview to previously uploaded PDF contracts at (x, y) coordinates with desired height to
desired page on pdf. Coordinates are relative to left bottom corner of page. Every document id can have multiple signature
attached to it, hence the array of signature positions for every document.

When the session is linked to an identity (successful face authentication, or an approved session) the DEFAULT
signature stored on that identity is used. The signature captured in the session itself is used only when the
session has no linked identity, or that identity has no stored signature.