Add files via upload

[danielatgoogle]

Hello DevRel friends!

Description

Working with @gtondello, I'm uploading code samples for the Google Chat Advanced Service in Apps Script launch.

Fixes # (issue)

Is it been tested?

• Development testing done

Checklist

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

[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• advanced/chat.gs:17, tag chat_post_message_with_user_credentials
• advanced/chat.gs:33, tag chat_post_message_with_app_credentials
• advanced/chat.gs:57, tag chat_get_space
• advanced/chat.gs:74, tag chat_create_space
• advanced/chat.gs:89, tag chat_list_memberships

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