[WIP] Add Kafka event sourcing for Go functions

[aliok]

WIP until knative-extensions/func-go#185 is merged

Summary

• Add run.kafka config to func.yaml (brokers, topic, consumerGroup)
  instead of a separate invoke: kafka type
• Deployer and runner automatically set FUNC_TRANSPORT=kafka plus
  KAFKA_BROKERS, KAFKA_TOPIC, KAFKA_CONSUMER_GROUP env vars from the
  config — user never sets these manually
• Scaffolding main.go branches at runtime between kafka.Start() and
  ce.Start() based on FUNC_TRANSPORT
• Add Kafka consumption docs to CloudEvents template READMEs
• Add fmt.Println(e) to instanced CloudEvents template to match static
  template behavior

How it works

Users add a run.kafka section to their existing CloudEvents function:

invoke: cloudevent
run:
  kafka:
    brokers: "my-cluster-kafka-bootstrap.kafka:9092"
    topic: "my-topic"
    consumerGroup: "my-function-group"

No code changes — the same Handle(event.Event) method receives Kafka
messages automatically converted to CloudEvents by the runtime.

Dependencies

• Requires func-go #185

Limitations

• Go only

Supersedes #3923

Closes #3940

[knative-prow]

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[Copilot]

Pull request overview

This PR introduces Kafka-backed event consumption for Go CloudEvents functions by adding a run.kafka configuration block (brokers/topic/consumerGroup) and wiring deploy/run paths to select Kafka transport at runtime via environment variables.

Changes:

• Adds run.kafka to the function model, validation, and func.yaml JSON schema.
• Injects FUNC_TRANSPORT=kafka plus KAFKA_* env vars from run.kafka in the runner and Kubernetes/Knative deployers.
• Updates Go CloudEvents templates/scaffolding and READMEs to support/describe Kafka consumption, and aligns instanced template logging with static template behavior.

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
templates/go/scaffolding/static-cloudevents/main.go	Switches runtime start between CloudEvents HTTP and Kafka based on FUNC_TRANSPORT.
templates/go/scaffolding/instanced-cloudevents/main.go	Same transport switch for instanced CloudEvents functions.
templates/go/cloudevents/README.md	Adds Kafka consumption documentation for the Go CloudEvents template.
templates/go/cloudevents/function.go	Adds event echo output to match static template behavior.
templates/go/.static-cloudevents/README.md	Adds Kafka consumption documentation for the static Go CloudEvents template.
schema/func_yaml-schema.json	Extends schema with run.kafka configuration (KafkaConfig).
pkg/scaffolding/signatures.go	Minor formatting/comment tweak to signature map.
pkg/scaffolding/signatures_test.go	Asserts kafka invoke/template remains unknown in scaffolding signature selection.
pkg/knative/deployer.go	Appends Kafka env vars when generating/updating Knative services.
pkg/k8s/deployer.go	Appends Kafka env vars during Kubernetes deployment generation.
pkg/k8s/deployer_test.go	Adds unit tests for Kafka env var injection helper.
pkg/functions/runner.go	Adds Kafka env var injection for local/host runner jobs.
pkg/functions/runner_test.go	Adds unit tests validating runner env injection behavior.
pkg/functions/function.go	Adds KafkaConfig, validation rules, and RunSpec.Kafka field.
pkg/functions/function_test.go	Adds YAML round-trip and omit-empty tests for Run.Kafka.
docs/reference/func_create.md	Updates generated CLI reference output to include a Go kafka template.

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