Allow using serve mode with system-internal-tls

[linkvt]

Hi,

related PR in networking: knative/networking#1093

Proposed Changes

• Allow using Serve mode with system-internal-tls
  • right now Proxy mode is always forced when using system-internal-tls
  • we should allow disabling this behavior as some ingresses support validating certificates with multiple SANs and do not need to hop over the activator

Release Note

Allow using Serve mode when using system-internal-tls

Questions:

1. Should this change result in a docs change, even if I marked it as alpha flag for now?

Thanks for the feedback!

/kind enhancement
/cc @Fedosin

[knative-prow]

Skipping CI for Draft Pull Request.
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[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.02%. Comparing base (1dc73ee) to head (98fda9c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16183      +/-   ##
==========================================
- Coverage   80.06%   80.02%   -0.05%     
==========================================
  Files         214      214              
  Lines       16940    16941       +1     
==========================================
- Hits        13563    13557       -6     
- Misses       3019     3023       +4     
- Partials      358      361       +3     

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