feat(ncu-config): add support for partially encrypted config files

[aduh95]

One frustration I have since I switched to an encrypted config file is that I have to type my passphrase when the tool doesn't need access to any sensitive information (e.g. my gh handle). This PR should solve this by only asking my passphrase when it's needed, telling me why it's needed.

[codecov]

Codecov Report

❌ Patch coverage is 56.60377% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.59%. Comparing base (2589b9c) to head (ea20ccf).

Files with missing lines	Patch %	Lines
lib/config.js	50.00%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1004      +/-   ##
==========================================
+ Coverage   79.72%   80.59%   +0.87%     
==========================================
  Files          39       39              
  Lines        4675     4725      +50     
==========================================
+ Hits         3727     3808      +81     
+ Misses        948      917      -31     

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