Fix #16298: Allow customization of headers in Route

[aviralgarg05]

Pull Request Description

Fixes #16298

Proposed Changes

• API Development: Added the AppendHeaders field (type map[string]string) to the TrafficTarget struct in pkg/apis/serving/v1/route_types.go. This allows users to specify custom HTTP headers for specific traffic targets in a Knative Route.
• Validation: Implemented validateAppendHeaders in pkg/apis/serving/v1/route_validation.go to ensure that all keys provided in AppendHeaders are valid HTTP header names according to RFC 7230 and that the field is not present in the Status of the Route.
• Ingress Propagation: Updated the Ingress generation logic in pkg/reconciler/route/resources/ingress.go. The AppendHeaders from the Route's TrafficTarget are now merged with system-level headers (like activator headers) using kmeta.UnionMaps and propagated to the IngressBackendSplit.
• DeepCopy Logic: Manually updated pkg/apis/serving/v1/zz_generated.deepcopy.go to include deep copy support for the new AppendHeaders map, ensuring the field survives reconciliation cycles.
• Unit Testing:
  • Added test cases to pkg/apis/serving/v1/route_validation_test.go to verify valid and invalid header configurations.
  • Added TestMakeIngressSpecWithAppendHeaders to pkg/reconciler/route/resources/ingress_test.go to confirm that headers are correctly passed through to the networking layer.

Release Note

Added `AppendHeaders` field to `TrafficTarget` in the Route API (v1) to allow specifying additional HTTP headers to be added before forwarding a request to the destination service.

[knative-prow]

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[knative-prow]

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This pull-request has been approved by: aviralgarg05
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.20%. Comparing base (2d3f345) to head (ea18c4a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16355      +/-   ##
==========================================
- Coverage   80.23%   80.20%   -0.03%     
==========================================
  Files         216      216              
  Lines       13440    13450      +10     
==========================================
+ Hits        10784    10788       +4     
- Misses       2293     2298       +5     
- Partials      363      364       +1     

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[linkvt]

Hi @aviralgarg05 ,
thanks for the PR, this topic is a bit more complicated though as the PR is only one side of solving this problem.
The other side is net-... implementations like https://github.com/knative-extensions/net-kourier that follow the specification to actually add the headers to the requests they route to the knative service.

I'm not experienced enough to know extending the spec would work especially considering that adding headers might not be supported by all implementations, I guess @dprotaso has more background on what could be done?