[SDK] HttpClient - Add Support for multipart/form-data

[ThuF]

Description

Currently, the HttpClient in Eclipse Dirigible’s SDK does not provide native support for sending requests with multipart/form-data (form parameters). This limits integration with APIs that require file uploads or form-based submissions. Adding first-class support for multipart/form-data would significantly improve the SDK’s usability for real-world scenarios such as:

• File uploads
• Form submissions
• Mixed payload requests (text + binary)

Expected Behavior

The HttpClient should support constructing and sending multipart/form-data requests, including:

• Text fields (key-value pairs)
• File uploads (binary data / streams)
• Automatic boundary handling
• Proper Content-Type header management

Example (pseudo-usage):

HttpClient.post("/upload", {
  formData: {
    name: "example",
    file: Files.createOutputStream("file.txt")
  }
});

Acceptance Criteria

• Ability to send multipart/form-data requests
• Support for both text and binary fields
• Automatic header handling
• Clear API documentation and examples