Refactor pipeline datasource and I/O modules (#21)

* Refactor pipeline datasource and IO argument modules.

This introduces a reusable DataSource abstraction and splits read/write argument structs into dedicated modules while updating convert tests to use DataFrameReader directly.

Made-with: Cursor

* Remove unused `convert` module and clean up CLI structure

This commit deletes the `convert` module from the CLI, streamlining the codebase by removing unnecessary components. The `repl` module remains intact, ensuring continued functionality for interactive command execution. Additionally, minor adjustments were made to the CLI structure for improved clarity and organization.

Author: aisrael

src/cli.rs

@@ -1,6 +1,5 @@
 //! Contains shared types for the `datu` CLI and implementation.
 
-pub mod convert;
 pub mod repl;
 
 use std::str::FromStr;