add aload_data to BaseReader and add implementation for SimpleDirectoryReader (#12156)

...cs/examples/data_connectors/simple_directory_reader.ipynb

...s/data_connectors/simple_directory_reader_remote_fs.ipynb

llama-index-core/llama_index/core/async_utils.py

llama-index-core/llama_index/core/readers/base.py

llama-index-core/llama_index/core/readers/file/base.py