Refactor OData handler

[habbes]

Discussed in #2465

^{Originally posted by habbes July 13, 2021}
Hi, I'd like to refactor the OData handler to make it easier to fix bugs and improvements. Currently the entire handler codebase is in a single file which makes it a bit difficult to navigate. There's also multiple instances of duplicate code and functions which could be easily reused to make it easier to maintain and fix bugs. I'd like to start off with splitting the code into separate modules. Then future refactoring work can be made alongside other bug fixes and improvements.