Add system-internal-tls-allow-serve-mode configuration option

[linkvt]

Hi,

related serving PR: https://github.com/knative/serving/pull/16183

this PR adds a new configuration field to allow Serve mode when system-internal-tls is enabled. By default, enabling system-internal-tls forces all traffic to use Proxy mode because not all Knative ingress implementations support Serve mode with TLS enabled.

The new system-internal-tls-allow-serve-mode option allows users to enable Serve mode when their ingress implementation supports direct TLS connections to application containers.

Configuration:

• Default: Disabled (forces Proxy mode for compatibility, current behavior)
• Enabled: Allows Serve mode even with TLS enabled

I added the option to the network config as it makes IMO sense to put it close to the relevant system-internal-tls option, even if the effect is not really related to it.

Changes

• :gift: Allow enabling Serve mode when using system-internal-tls (alpha)

/kind enhancement

Release Note

Allow enabling Serve mode when using system-internal-tls (alpha)
- Adds a new configuration option `system-internal-tls-allow-serve-mode` to allow 
Serve mode when system-internal-tls is enabled. This is useful for ingress 
implementations that support direct TLS connections to application containers (e.g. kourier or istio).
- This is an alpha feature. Only enable if your ingress implementation 
supports Serve mode with TLS.

Questions:

1. Do we want/need to stage this flag starting as alpha right now?
2. Is a release note in this PR something we want do for a) config options and b) alpha options?
3. Config option name is maybe a bit lengthy, let me know if I should improve this (possibly with suggestions).

Docs

Thanks for the feedback!

/cc @Fedosin

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 93.16%. Comparing base (0bde191) to head (94883fd). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

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+ Coverage   93.14%   93.16%   +0.02%     
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  Files          36       36              
  Lines        1255     1259       +4     
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+ Hits         1169     1173       +4     
  Misses         72       72              
  Partials       14       14              

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[Fedosin]

Hey! Thanks for the work you've done. The code is good, so I'm happy to put LGTM when you are ready.

Wrt your questions:

Do we want/need to stage this flag starting as alpha right now?

I think so, yes. The feature enables a potentially breaking behavior change for users whose ingress implementations don't support Serve mode with TLS, so this should be marked as alpha.

Is a release note in this PR something we want do for a) config options and b) alpha options?

For config options - yes, as release notes are valuable as they help users discover new configuration capabilities. For alpha options - also yes, but clearly mark as alpha/experimental.

AI suggested this release note:

Allow enabling Serve mode when using system-internal-tls (alpha)

Adds a new configuration option `system-internal-tls-allow-serve-mode` to allow 
Serve mode when system-internal-tls is enabled. This is useful for ingress 
implementations that support direct TLS connections to application containers.

Note: This is an alpha feature. Only enable if your ingress implementation 
supports Serve mode with TLS.

Config option name is maybe a bit lengthy, let me know if I should improve this (possibly with suggestions).

Imo, it's okay. If we really need something shorter, it can be allow-serve-mode-with-tls

[linkvt]

Thanks for the review, I updated the PR description and the other related PR.

/remove-approve - was added automatically as I'm co-release lead.

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[linkvt]

/check-cla

[linkvt]

Could you please take a look @dprotaso ?