Webhook EventSource support for alternative contentTypes

[opensean]

Does the webhook EventSource support contentTypes other than 'application/json'? For example, I have an external service that uses the 'application/x-www-form-urlencoded' contentType. I receive the following error when using the webhook EventSource, which looks like a parsing error because the webhook eventsource is expecting json:

{"level":"error","ts":1647980958.51209,"logger":"argo-events.eventsource","caller":"webhook/start.go:124","msg":"failed to construct the event payload","eventSourceName":"gfycore","eventSourceType":"webhook","eventName":"job","endpoint":"/job/","port":"12000","http-method":"POST","error":"json: error calling MarshalJSON for type *json.RawMessage: invalid character 'i' looking for beginning of value","stacktrace":"github.com/argoproj/argo-events/eventsources/sources/webhook.(*Router).HandleRoute\n\t/home/runner/work/argo-events/argo-events/eventsources/sources/webhook/start.go:124\ngithub.com/argoproj/argo-events/eventsources/common/webhook.startServer.func2\n\t/home/runner/work/argo-events/argo-events/eventsources/common/webhook/webhook.go:148\nnet/http.HandlerFunc.ServeHTTP\n\t/opt/hostedtoolcache/go/1.17.1/x64/src/net/http/server.go:2046\ngithub.com/gorilla/mux.(*Router).ServeHTTP\n\t/home/runner/go/pkg/mod/github.com/gorilla/mux@v1.8.0/mux.go:210\nnet/http.serverHandler.ServeHTTP\n\t/opt/hostedtoolcache/go/1.17.1/x64/src/net/http/server.go:2878\nnet/http.(*conn).serve\n\t/opt/hostedtoolcache/go/1.17.1/x64/src/net/http/server.go:1929"}

I see the slack eventsource has some support for the 'x-www-form-urlencoded' content type here. Would support of the 'x-www-form-urlencoded' contentType on a webhook require a custom eventsource implementation?