Do not create FastAPI app until config loaded

[DiamondJoseph]

Defers creation of the FastAPI app, which will allow configuration to be loaded first.
This will allow us to e.g. configure authentication prior to starting the app.

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.32%. Comparing base (1c50a4b) to head (1ed5c2e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/blueapi/service/main.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #702      +/-   ##
==========================================
- Coverage   92.35%   92.32%   -0.04%     
==========================================
  Files          35       35              
  Lines        1793     1799       +6     
==========================================
+ Hits         1656     1661       +5     
- Misses        137      138       +1     

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