Add validation for activator-ca and activator-san

[nak3]

This is a follow up for #608.

This patch adds a validation to disallow activator-ca without activator-san.
I think we can relax the validation in the future if users want to skip SAN validation
but the initial implementation should be strict.

/cc @evankanderson @ZhiminXiang @carlisia

[codecov]

Codecov Report

Merging #637 (1f27f8a) into main (f35c7e7) will decrease coverage by 0.22%.
The diff coverage is 50.00%.

❗ Current head 1f27f8a differs from pull request most recent head 8e37be9. Consider uploading reports for the commit 8e37be9 to get more accurate results

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
- Coverage   94.78%   94.56%   -0.23%     
==========================================
  Files          38       38              
  Lines         806      810       +4     
==========================================
+ Hits          764      766       +2     
- Misses         30       31       +1     
- Partials       12       13       +1     

Impacted Files	Coverage Δ
pkg/network.go	85.91% <50.00%> (-1.05%)	⬇️

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[evankanderson]

/lgtm
/approve

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