Refactor murfey.server.api into submodules

[tieneupin]

Currently, the main API module murfey.server.api has exceeded 1600 lines, and API endpoints for auth, clem, and spa are stored separately in their own sub-folders.

I suggest refactoring the API endpoints to take the following file structure as an attempt to improve organisation:

src/murfey
|__ server
    |__ api
        |__ __init__.py  <- Stores unsorted API endpoints from what was formerly 'murfey/server/api.py'
        |__ auth.py
        |__ clem.py
        |__ spa.py
        |__ ...  <- We can slowly migrate items from the __init__.py file based on themes