Convert Slack client project to a multi-module Maven project

[ArturSlimak]

🚀 Pull Request

📌 Summary

I restructured the project into a multi-module setup to separate concerns and make the core Slack client framework-agnostic. This avoids forcing any Spring Boot dependencies on users who just want to use the plain Java client. They won't need to to set

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot</artifactId>
    <version>4.0.3</version>
    <scope>compile</scope>
</dependency>

this only

<dependency>
    <groupId>com.n1netails</groupId>....</dependency>

🧪 What Does This Fix or Improve?

• Bug fix
• New feature
• Performance improvement
• Documentation update
• Other (describe below)

🔍 Changes

List the notable changes:

• structure of the project

✅ Checklist

Please ensure the following:

• Code compiles without errors
• Tests pass successfully
• No breaking changes introduced
• Added or updated documentation if needed
• Branch is up to date with main

🧪 Testing Steps

Explain how reviewers can test your changes.

📸 Screenshots (if applicable)

Attach images or logs here.

🔗 Related Issues