refactor: Modify service override registration so they're mapped to a "service"

[ollieread]

This PR addresses #84 with a refactor so that service overrides are registered against services, which are arbitrary strings that represent the “service” they override.

[codecov]

Codecov Report

Attention: Patch coverage is 47.05882% with 9 lines in your changes missing coverage. Please review.

Project coverage is 54.19%. Comparing base (23b4d58) to head (82d431d).
Report is 7 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/Concerns/HandlesServiceOverrides.php	38.46%	8 Missing ⚠️
src/SproutServiceProvider.php	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #85      +/-   ##
============================================
+ Coverage     54.04%   54.19%   +0.14%     
- Complexity      476      484       +8     
============================================
  Files            65       65              
  Lines          1347     1360      +13     
============================================
+ Hits            728      737       +9     
- Misses          619      623       +4     

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