triggers: Support trigger via CloudEvents

Feature request

Support CloudEvents compliant responses.

I see a few options:

• When a cloudevent is received, respond with a cloudevent. The current body could be placed in the payload.
• When a cloudevent is received, respond with no payload
• Allow users to disable the response payload platform wise

Use case

When the project was started, it was decided to support generic HTTP requests with JSON payload rather than focus specifically on CloudEvents. The current behaviour of triggers is not compliant with the CloudEvents spec though, which makes it hard to use Triggers as a sink for CloudEvents.

When using triggers with Knative broker, the broker checks if the response is a cloud event, or empty, and reports a 502 back otherwise. See the code and the logs from a test cluster:

{"level":"debug","ts":"2022-09-01T10:59:16.299Z","logger":"mt_broker_filter","caller":"filter/filter_handler.go:173","msg":"Received message","commit":"56edb3b","triggerRef":"default/all-to-tekton-el-events"}
{"level":"debug","ts":"2022-09-01T10:59:16.299Z","logger":"mt_broker_filter","caller":"filter/filter_handler.go:353","msg":"Applying attributes filter.","commit":"56edb3b","trigger":"default/all-to-tekton-el-events","filter":{}}
{"level":"debug","ts":"2022-09-01T10:59:16.350Z","logger":"mt_broker_filter","caller":"filter/filter_handler.go:222","msg":"Successfully dispatched message","commit":"56edb3b","target":"http://el-events.default.svc.cluster.local:8080"}
{"level":"error","ts":"2022-09-01T10:59:16.350Z","logger":"mt_broker_filter","caller":"filter/filter_handler.go:227","msg":"failed to write response","commit":"56edb3b","error":"received a non-empty response not recognized as CloudEvent. The response MUST be either empty or a valid CloudEvent","stacktrace":"knative.dev/eventing/pkg/broker/filter.(*Handler).send\n\tknative.dev/eventing/pkg/broker/filter/filter_handler.go:227\nknative.dev/eventing/pkg/broker/filter.(*Handler).ServeHTTP\n\tknative.dev/eventing/pkg/broker/filter/filter_handler.go:209\ngo.opencensus.io/plugin/ochttp.(*Handler).ServeHTTP\n\tgo.opencensus.io@v0.23.0/plugin/ochttp/server.go:92\nknative.dev/pkg/network/handlers.(*Drainer).ServeHTTP\n\tknative.dev/pkg@v0.0.0-20220524202603-19adf798efb8/network/handlers/drain.go:110\nnet/http.serverHandler.ServeHTTP\n\tnet/http/server.go:2879\nnet/http.(*conn).serve\n\tnet/http/server.go:1930"}