[WIP] Add Kafka invoke type for Go functions

[aliok]

NOTE: THIS IS A POC. Do not review the whole thing, but review the overall structure and the integration points.

Add Kafka middleware support to the scaffolding system, enabling Go
functions that consume Kafka messages. Adds instanced and static
templates, signature detection, and scaffolding that wires user
functions to knative.dev/func-go/kafka.

Related middleware changes: knative-extensions/func-go#169

[knative-prow]

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[knative-prow-robot]

PR needs rebase.

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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	b2688f4	link	true	/test unit-tests

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