[WIP] Add Kafka invoke type for Go functions

[aliok]

WIP until knative-extensions/func-go#169 is merged

Summary

• Add kafka as a new invoke type for Go functions
• Add instanced and static Kafka templates (templates/go/.instanced-kafka/, templates/go/kafka/)
• Add scaffolding that wires user functions to knative.dev/func-go/kafka
• Add InstancedKafka and StaticKafka signature detection in pkg/scaffolding/signatures.go
• Allow invoke: kafka in func.yaml

How it works

The Kafka middleware connects directly to Kafka brokers as a consumer group
member — no Knative Eventing or CloudEvents required. The function receives
raw kafka.Message structs (key, value, headers as []byte).

Configuration is done via environment variables (KAFKA_BROKERS,
KAFKA_TOPICS, KAFKA_CONSUMER_GROUP), which can be set as plain values,
from Kubernetes Secrets, or from ConfigMaps using the existing func.yaml
envs syntax:

envs:
- name: KAFKA_BROKERS
  value: "{{ secret:kafka-config:brokers }}"
- name: KAFKA_TOPICS
  value: "{{ configMap:kafka-config:topics }}"
- name: KAFKA_CONSUMER_GROUP
  value: "my-group"

Delivery semantics

• At-least-once, in-order per partition
• No retry, no deduplication, no exactly-once (planned as future work)

Dependencies

• Requires func-go #169 to be merged and released first
• Template go.mod files reference knative.dev/func-go v0.21.3 which will need a version bump once func-go is released with the kafka package

Limitations

• Go only — no support for other languages yet

[knative-prow]

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[knative-prow]

@aliok: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
unit-tests_func_main	73a69cf	link	true	/test unit-tests

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