feat: Add Google Chat API quickstart

[PierrickVoulet]

Description

Add the code sample that will be used in the quickstart guide for Google Chat API in Google Developers Website.

Is it been tested?

• [x] Development testing done

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have performed a peer-reviewed with team member(s)
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [chat/quickstart/Code.gs:17](https://github.com/PierrickVoulet/apps-script-samples/blob/b4a202cf1c81ac042ba0f7efe9e8411f08fcf1d5/chat/quickstart/Code.gs#L17), tag `chat_quickstart`

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[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/