Slack Templates (Raw & Blocks API JSON)

[operatorequals]

Description:

Related issue (if applicable): #641

This PR adds the following functionality to the Slack plugin
(Similar to MSTeams implementation)

• template key to all Slack URLs accepts a file (or any attachment) which is used to generate the sent message

• Tokens prepended with : are dynamically added to the template (e.g: ...&template=file.txt&:token=value)

• Full Support of the Block Kit API
  The templates can utilize the full power of the Blocks Kit API. If the AppRise URL contains blocks=yes, then the template file is parsed as JSON, and it is passed as-is. An example follows:
  template-blocks.json

{
  "blocks": [
    {
      "type": "section",
      "text": {
        "type": "mrkdwn",
        "text": "Hey there 👋: {{ app_desc }}."
      }
    },
    {
      "type": "section",
      "text": {
        "type": "mrkdwn",
        "text": "{{ app_body }}"
      }
    },
    {
      "type": "actions",
      "elements": [
        {
          "type": "button",
          "text": {
            "type": "plain_text",
            "emoji": true,
            "text": "{{ app_id }}"
          },
          "style": "primary",
          "value": "click_me_123"
        }
      ]
    }
  ]
}

apprise_url = 'slack://x/x/x?blocks=true&template=template-blocks.json'
apprise_obj = apprise.Apprise()
apprise_obj.add(apprise_url)

apprise_obj.notify(body="body of the message", title="TITLE")

NOTE: As you can see the title is not rendered, as it was not used in the template. With great power comes great responsibility.

NOTE: footer and params that traditionally modify the message are ignored when template is used with blocks.

• If the URL does not contain blocks param, then the template is free text, only sent as body of the notification. The title is processed as usually.

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

Testing

This PR's feature is testable by everyone!

We can use a Github Gist containing a template!

The Gist below contains the Blocks template used above:
https://gist.github.com/operatorequals/16864f51c215a0a9628f934b0740095a
We can use it by filling its Raw URL to the AppRise template, getting an AppRise URL as below:

slack://x/x/x?blocks=true&template=https://gist.githubusercontent.com/operatorequals/16864f51c215a0a9628f934b0740095a/raw/fd582669e3e35f9a9705946568989f41a3fe8166/slack-blocks-apprise-template.json

NOTE: fill the x to your Slack Incoming Webhook values

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@<this.branch-name>

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  <apprise url related to ticket>