[WIP] Use cert-manager to deploy internal certificates

[nak3]

This patch allows to use cert-manager to deploy internal certificates.

The implementation does not have any change actually but it changes to:

• Remove the script to generate cert (test/generate-cert.sh) as cert-manager does everything.
• Fix Support encrypted traffic between ingress and queue-proxy #12797.

TODO:

• Allow to specify the namespace for CA secret. (cert-manager needs to deploy CA in cert-manager ns.)
• Allow to specify the CA cert key. (cert-manager uses tls.crt instead of ca.crt).

[knative-prow]

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[codecov]

Codecov Report

Merging #12998 (5b0beb3) into main (bb5bdbb) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main   #12998      +/-   ##
==========================================
- Coverage   87.01%   86.99%   -0.02%     
==========================================
  Files         197      197              
  Lines       14439    14440       +1     
==========================================
- Hits        12564    12562       -2     
- Misses       1580     1583       +3     
  Partials      295      295              

Impacted Files	Coverage Δ
cmd/activator/main.go	0.00% <0.00%> (ø)
pkg/reconciler/autoscaling/kpa/kpa.go	95.21% <ø> (-0.06%)	⬇️

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[nak3]

Closing as I think we should start narrow support like #13005 instead of cert-manager support.