fix: update UseServerCertificateSelector to call the original selector

[natemcmaster]

Resolves #122

This updates the default configuration of LettuceEncrypt to fallback to any existing certificate selector which may already be configured in Kestrel.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ba32cda) 44.30% compared to head (63feb2e) 44.50%.

Files	Patch %	Lines
...trel.Certificates/KestrelHttpsOptionsExtensions.cs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   44.30%   44.50%   +0.19%     
==========================================
  Files          51       51              
  Lines        1167     1173       +6     
  Branches      130      131       +1     
==========================================
+ Hits          517      522       +5     
  Misses        613      613              
- Partials       37       38       +1     

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