age-plugin: Fix run_state_machine to enable recipient-only or identity-only plugins

[str4d]

Closes #494.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 51.41%. Comparing base (a510e76) to head (2eec457).

Files	Patch %	Lines
age-plugin/src/lib.rs	0.00%	11 Missing ⚠️
age-plugin/src/recipient.rs	0.00%	6 Missing ⚠️
age-plugin/src/identity.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #511      +/-   ##
==========================================
- Coverage   51.57%   51.41%   -0.16%     
==========================================
  Files          42       42              
  Lines        3938     3946       +8     
==========================================
- Hits         2031     2029       -2     
- Misses       1907     1917      +10     

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