Added Google Chat Support

[caronc]

Description:

Related issue (if applicable): #344

Setup

For this to work correctly you need to create a webhook. You'll also
need a GSuite account (there are free trials if you don't have one)

• Open Google Chat in your browser:
  Link: https://chat.google.com/
• Go to the room to which you want to add a bot.
• From the room menu at the top of the page, select Manage webhooks.
• Provide it a name and optional avatar and click SAVE
• Copy the URL listed next to your new webhook in the Webhook URL column.
• Click outside the dialog box to close.

When you've completed, you'll get a URL that looks a little like this:

• https://chat.googleapis.com/v1/spaces/AAAAk6lGXyM/messages?key=AIzaSyDdI0hCZtE6vySjMm-WEfRq3CPzqKqqsHI&token=O7b1nyri_waOpLMSzbFILAGRzgtQofPW71fEEXKcyFk%3D

Simplified, it looks like this:

• https://chat.googleapis.com/v1/spaces/WORKSPACE/messages?key=WEBHOOK_KEY&token=WEBHOOK_TOKEN

Syntax

This plugin will simply work using the urls of:

• gchat://{workspace}/{key}/{token}
• https://chat.googleapis.com/v1/spaces/{workspace}/messages?key={key}&token={token}
  • Hence, you can just pass in the generated webhook as is straight into Apprise. It will know what to do.

New Service Completion Status

• apprise/plugins/NotifyGoogleChat.py
• setup.py
  • add new service into the keywords section of the setup() declaration
• README.md
  • add entry for new service to table (as a quick reference)
• packaging/redhat/python-apprise.spec
  • add new service into the %global common_description

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage