Complete the Task File API implementation with:
- GetTaskFile: retrieve single file metadata by ID
- RemoveAllTaskFiles: remove all files attached to a task
- Add Username/UserName fields to TaskFile struct
- Add TaskScope convenience methods: GetFile, RemoveFile,
DownloadFile, RemoveAllFiles
- Comprehensive tests for all new methods
Closes: kanboard-amh
Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.
- GetAllTaskFiles: retrieve all files attached to a task
- CreateTaskFile: upload file with base64 encoding
- DownloadTaskFile: download file with base64 decoding
- RemoveTaskFile: delete a task file
- TaskScope.GetFiles and UploadFile for fluent API
- Automatic base64 encoding/decoding for file content
- Comprehensive test coverage
