Move certificate parameters to a dedicated struct

[yoavamit]

When generating a certificate or a certificate signing request, instead of getting the certificate subject information directly from environment variables, they'll be passed to the GenerateCertificate function in a dedicated struct.
The environment variables will still be used by the pivit CLI executable to populate the CertificateParameters struct when calling pivit --generate.

[linuxwolf]

🔑

[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.

Project coverage is 62.73%. Comparing base (51d77d0) to head (c616c6a).
Report is 6 commits behind head on main.

Files	Patch %	Lines
pkg/pivit/generate.go	89.28%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   63.22%   62.73%   -0.50%     
==========================================
  Files          10       10              
  Lines         756      754       -2     
==========================================
- Hits          478      473       -5     
- Misses        212      214       +2     
- Partials       66       67       +1     

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