Enhance Type Safety with better Types

[NewtTheWolf]

Description:

Enhance the type safety of the sendblue library by incorporating more robust types such as chrono for date handling, url::Url for URL validation, and creating custom types for parameters and other fields where necessary. This will improve code readability, maintainability, and reduce the likelihood of runtime errors.

Acceptance Criteria:

• Replace string representations of dates and times with chrono types.
• Replace string representations of URLs with url::Url types.
• Implement custom types for parameters that require specific validation or formatting.
• Update documentation and examples to reflect the changes.
• Add comprehensive tests to ensure the new types function as expected.