Sample App: Adding the knative function build for the sentiment analysis service (knative#5904)

* Adding the knative function build for the sentiment analysis service

* Update the sample code, so that the returned result is a cloudEvent

* Update the sample code to give a specific event type to the response cloudEvent

* Update the tutorial doc

* Update the tutorial doc

* Remove the docker registry info

* Fix nit

* Modify the return response type and how python function handle the incoming cloudEvent

* Adding the explaination for serving

* Unhide the alert box portion

* Remove the intentional delay

* Remove the duplicated line

* Make the input as json instead of plaintext

* Update the version of cloudEvent and update the deployment instruction

* Update the tutorial to use the public URL instead of cluster-IP

* Display the input text in the response

Author: Leo6Leo

code-samples/eventing/bookstore-sample-app/ML-sentiment-analysis/.funcignore

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+
+# Use the .funcignore file to exclude files which should not be
+# tracked in the image build. To instruct the system not to track
+# files in the image build, add the regex pattern or file information
+# to this file.

code-samples/eventing/bookstore-sample-app/ML-sentiment-analysis/.gitignore

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+
+# Functions use the .func directory for local runtime data which should
+# generally not be tracked in source control. To instruct the system to track
+# .func in source control, comment the following line (prefix it with '# ').
+/.func

code-samples/eventing/bookstore-sample-app/ML-sentiment-analysis/Procfile

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+web: python -m parliament .