[wip] Prevent duplicate hosts in Ingress rules when using ACME

[linkvt]

The basic idea is that we don't create ingress rules with multiple hosts for external visibility, as we need to be able merge ACME rules into these hosts.
I don't really like that though as it puts the knowledge of exactly 1 host per external rule in two places (rule creation and ACME rule merging).

Another alternative would be that we accept the incorrect rule merging with multiple hosts that we had before:

rules:
  - hosts: a, b
    paths:
      - acme-of-a
      - acme-of-b

This would result in 4 possible routes in the Ingress:

• host a with path acme-of-a - correct
• host b with path acme-of-a - wrong
• host a with path acme-of-b - wrong
• host b with path acme-of-b - correct

This won't be a problem in practice though as the ACME server would not try to do a request to host a with path acme-of-b.
This would be a simple loop over existing rules, check if it contains a matching host and merge it inside there (while still preventing double duplicate rules that we had before #16259 )

Let me know what you think of this approach in general.

/cc @dprotaso

Proposed Changes

Release Note

[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.06%. Comparing base (1e52818) to head (b08bcab).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16312      +/-   ##
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- Coverage   80.07%   80.06%   -0.02%     
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  Files         215      215              
  Lines       13361    13374      +13     
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+ Hits        10699    10708       +9     
  Misses       2304     2304              
- Partials      358      362       +4     

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