Git Commit Message Generator

**Context**: 
Git commit messages are a crucial part of version control, providing a clear and concise description of changes made to the codebase. A well-structured commit message helps developers understand the purpose and impact of each commit, making it easier to navigate the project history, identify bugs, and collaborate with team members. The goal is to create a generator that can produce high-quality, standardized commit messages based on input parameters.

**Detailed Instructions**: 
Design a Git commit message generator that takes in specific details about the commit and generates a message following standard professional guidelines. The generator should accept the following input parameters:
- **Type**: The type of commit (e.g., feature, fix, docs, style, refactor, perf, test, build, ci, chore, revert).
- **Brief Description**: A short summary of the changes made.
- **Detailed Description**: An optional longer description of the changes, which may include bullet points or short paragraphs.
- **Related Issues**: References to any related issues or pull requests (e.g., fixes #123, closes #456).
The generator should then produce a commit message that adheres to the conventional commit message format, which typically includes a type, a brief description, and an optional body for more details. Ensure the message is concise, clear, and properly formatted.

**Output Format**: 
The output should be a string representing the generated commit message. The format should follow the conventional commit message guidelines:
```
type: brief description

detailed description

related issues
```
If there is no detailed description or related issues, those sections should be omitted.

**Examples**: 
1. Input: 
   - Type: feature
   - Brief Description: Add user authentication
   - Detailed Description: Implemented login and registration functionality using JWT.
   - Related Issues: fixes #10, closes #20
   Output: 
   ```
feature: Add user authentication

Implemented login and registration functionality using JWT.

fixes #10, closes #20
```
2. Input: 
   - Type: fix
   - Brief Description: Resolve database connection issue
   - Detailed Description: None
   - Related Issues: fixes #5
   Output: 
   ```
fix: Resolve database connection issue

fixes #5
```