Add parsing functionality for time periods with tests in Rust This commit introduces a new feature for parsing strings representing time periods into a Period enum with variants for days, weeks, months, and years. The Period enum also includes a method to convert these periods in chrono::Duration objects. The parsing is done using the nom crate, with support for both numeric and written numbers. Additionally, unit tests are included to ensure the parsing works correctly for various inputs, includin edge cases and invalid strings.