docs(samples): added webhook sample

[galz10]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/webhook.py:16, tag dialogflow_cx_webhook

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[busunkim96]

Hi @galz10,

I chatted with some other Python samples owners during our office hours today about the dict/proto approach. (Thanks to @nicain for shedding more light on this as well!)

Given the unique nature of this feature (https://cloud.google.com/dialogflow/cx/docs/concept/webhook), we'll leave it up to the Product DPEs as to the best way to present it in samples.

Here are the pros/cons we discussed for each:

Proto Messages:

• Pro: Able to validate request/response message against proto definition prior to it being sent to API.
• Pro: Easier discovery of other fields/options when working through an IDE.
• Con: Harder to work with for someone unfamiliar with GAPIC client. May be unnecessarily complex for simple request/response construction.

Dictionary:

• Pro: More familiar format for folks who haven't worked with GAPIC libraries.
• Pro: Dictionaries map naturally to request/response JSON objects.
• Con: Increased chance of errors when messages become larger (have many fields set).