Allow HTTP->HTTPS redirection on Istio Gateway

[penguinush]

Extends #1482.

One of the ServerTLSSettings on Istio Gatway is httpsRedirect that, according to docs, redirects HTTP traffic to HTTPS. Usually this is a desired behaviour for production setup, the only alternative would be to not create HTTP listener at all.

PR leverages combination of nullable property and oneOf with required fields - this works fine currently as there is 2 distinct options (mode: <>, which requires credentialName to be set, or httpsRedirect, which is mutually exclusive with every other option), but it might become cumbersome if more options of different combinations are added.

Tested with the following specs:

spec:
  selector:
    istio: ingressgateway
  servers:
  - hosts:
    - '*.example.com'
    port:
      name: http
      number: 80
      protocol: HTTP
    tls:
      httpsRedirect: true
  - hosts:
    - '*.example.com'
    port:
      name: https-production
      number: 443
      protocol: https
    tls:
      credentialName: example-tls
      mode: SIMPLE

and

spec:
  selector:
    istio: ingressgateway
  servers:
  - hosts:
    - '*.example.com'
    port:
      name: http
      number: 80
      protocol: HTTP
  - hosts:
    - '*.example.com'
    port:
      name: https-production
      number: 443
      protocol: https
    tls:
      credentialName: example-tls
      mode: SIMPLE

Both produce correct Gateways.

Kubernetes 1.30 (EKS), Knative Operator 1.15.4, Knative Serving 1.15.0, Istio 1.23.0.

Proposed Changes

• Allows HTTP listener on Istio Gateway to be configured with automatic HTTP->HTTPS redirection.

Release Note

feature: Istio Gateway can be configured with automatic HTTP->HTTPS redirection

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: penguinush / name: Constantine (a194e47)

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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 66.31%. Comparing base (5e034b0) to head (a194e47).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1888   +/-   ##
=======================================
  Coverage   66.31%   66.31%           
=======================================
  Files          53       53           
  Lines        2084     2084           
=======================================
  Hits         1382     1382           
  Misses        587      587           
  Partials      115      115           

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