[SDL 0344] Use Promises in Server Logic of SDL Policy Server

[theresalech]

Proposal: Use Promises in Server Logic of SDL Policy Server

This proposal aims to change the majority of the server application logic from using callback-based to Promise-based asynchronous function management. This is an effort to modernize the codebase after years of stagnant structural changes to make the project more readable and easier to update when new features are requested.

Review: smartdevicelink/sdl_evolution#1177