feat: remove the need to call set_manager to set up

[bdraco]

This makes it a bit easier to use outside of ha since everything can be set up with

await habluetooth.BluetoothManager().async_setup()

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.84%. Comparing base (70b2f69) to head (125f95b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/habluetooth/manager.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
- Coverage   83.97%   83.84%   -0.13%     
==========================================
  Files          12       12              
  Lines        1198     1201       +3     
  Branches      138      139       +1     
==========================================
+ Hits         1006     1007       +1     
- Misses        144      145       +1     
- Partials       48       49       +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #130 will not alter performance

_{Comparing setup_onecall (125f95b) with main (70b2f69)}

Summary

✅ 3 untouched benchmarks