Refactor extract_txt_flow using unifed read_file function

[SayaZhang]

1. Refactor extract_txt_flow and update extract_txt example using unified read_file function
2. Delete extract_txt_s3_flow and update extract_txt_from_s3 example
3. Refactor read_file function
4. Delete extract_markdown_flow since using extract_md_flow

[CambioML]

Approved with minor comment.

[notion-workspace]

[Uniflow] Unified read_file function example