Reqwest blocking feature

[ColinRhys]

Pull Request for Issue - Allow library user to use the library synchronously
This pull request introduces the "blocking" feature to the festivus crate, allowing users to opt for synchronous execution of the calculate_fee function.

Changes Made:

• Implemented a synchronous version of the calculate_fee function, which can be activated using the "blocking" feature flag.
• Maintained the same function signature for both asynchronous and synchronous versions of calculate_fee. The appropriate version is determined by the feature flag:
• #[cfg(not(feature = "blocking"))] for the asynchronous (default) version.
• #[cfg(feature = "blocking")] for the synchronous version.
• Updated the reqwest dependency in Cargo.toml to include the "blocking" feature.
• Added new tests to ensure the functionality of the synchronous version of calculate_fee.
• Updated the README file to include instructions on how to use the new "blocking" feature.

Branching Strategy:
Branched off from the branch containing changes to the calculate_fee function signature in the other pull request I created to maintain consistency and avoid merge conflicts.