refactor(crate): split library exports into lib.rs

[shivv23]

Summary

This PR refactors crate structure to make rustcloud a proper library crate surface and removes manifest noise that can confuse builds/tooling.

Changes

• Added src/lib.rs as the canonical module export surface.
• Simplified src/main.rs to a minimal binary entrypoint.
• Removed invalid Cargo.toml entries:
  • self-referential dev dependency on rustcloud
  • editor-only [[language]] block
• Removed stale module declarations pointing to missing files in AWS modules.
• Fixed gcp_storage response handling for reqwest 0.12 (capture status before consuming response body).

Why

• Improves crate correctness and downstream usability as a library.
• Removes non-Cargo metadata from manifest.
• Avoids compile issues caused by missing module declarations and moved Response values.

Validation

• cargo check --lib