Refactor/move constants to params

[JuanVP2]

Closes #1572

Summary

This PR refactors bill_payments by extracting operational constants and parameters from src/lib.rs into a dedicated src/params.rs module.

Changes Made

• Created bill_payments/src/params.rs containing all configuration limits, operational constants, and rate limiting parameters.
• Updated bill_payments/src/lib.rs to declare pub mod params; and re-export pub use params::*; ensuring backward compatibility.
• Maintained exact runtime behavior and module interfaces without logic changes.

Verification

• cargo check -p bill_payments
• cargo test --lib -p bill_payments