Refactor Slack API Hook and add new Connection type

[Taragolis]

Refactor Slack API hook

1. Retrieve token and create client outside of __init__ method
2. Create new connection type. Right now only Slack Webhook connection exists.
3. Add test method to connection.
4. Customise Slack API Connection UI.
5. Show deprecation warning if user provide token in Hook argument. IMHO, there is not a good idea provide directly token better to use Connection.
6. Keep compatible with generic connection.
7. Mask token provided as hook argument.

Connection UI and Testing Connection

[Taragolis]

This additional indent required to proper formatted block in documentation and PyCharm

Without additional space
https://airflow.apache.org/docs/apache-airflow-providers-slack/stable/_api/airflow/providers/slack/hooks/slack/index.html#airflow.providers.slack.hooks.slack.SlackHook

With additional space

Might be possible avoid this "hack"?

[Taragolis]

Also might be better just remove this method.
It not used after changes and it is hardly possible that someone override it

[Taragolis]

I create separate class for use in further steps

1. Refactor airflow.providers.slack.hooks.slack_webhook.SlackWebhookHook which mostly designed for use with legacy Slack Integrations
2. Create new hook and connection which will use slack_sdk.webhook.WebhookClient and create integration with Slack Incoming Webhook based on Slack API/App