WIP: Slack compatiblity with its new 2024 API

[caronc]

Description:

Related issue (if applicable): #1212

Support the new Slack API which will officially deprecate the old on Mar 31st, 2025 (coming soon)

Note: This is very much a heavy work in progress and in it's current state does not work. Just committing to get code off my PC

Seeing how the Slack plugin is one of the earlier Apprise plugins, it's structure was rather bloated, so this PR will also involve it's refactoring.

Despite time running out, the adaptation made in this PR will remain compatible with the old API with a deprecation warning to those still using it (which is everybody 😉 ) alerting them to convert.

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

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@1212-new-slack-api-support

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  "slack://credentials"