feat(slack): add Slack App integration (#2305)

Co-authored-by: Pepe Fagoaga <pepe@verica.io>

docs/tutorials/integrations.md

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+# Integrations
+
+## Slack
+
+Prowler can be integrated with [Slack](https://slack.com/) to send a summary of the execution having configured a Slack APP in your channel with the following command:
+
+```sh
+prowler <provider> --slack
+```
+
+![Prowler Slack Message](img/slack-prowler-message.png)
+
+> Slack integration needs SLACK_API_TOKEN and SLACK_CHANNEL_ID environment variables.
+### Configuration
+
+To configure the Slack Integration, follow the next steps:
+
+1. Create a Slack Application:
+    - Go to [Slack API page](https://api.slack.com/tutorials/tracks/getting-a-token), scroll down to the *Create app* button and select your workspace:
+    ![Create Slack App](img/create-slack-app.png)
+
+    - Install the application in your selected workspaces:
+    ![Install Slack App in Workspace](img/install-in-slack-workspace.png)
+
+    - Get the *Slack App OAuth Token* that Prowler needs to send the message:
+    ![Slack App OAuth Token](img/slack-app-token.png)
+
+2. Optionally, create a Slack Channel (you can use an existing one)
+
+3. Integrate the created Slack App to your Slack channel:
+    - Click on the channel, go to the Integrations tab, and Add an App.
+    ![Slack App Channel Integration](img/integrate-slack-app.png)
+
+4. Set the following environment variables that Prowler will read:
+    - `SLACK_API_TOKEN`: the *Slack App OAuth Token* that was previously get.
+    - `SLACK_CHANNEL_ID`: the name of your Slack Channel where Prowler will send the message.