Add `reqwest:blocking` feature

[bennyhodl]

Festivus uses the mempool.space api for getting the recommended fees. This in turn makes the calculate_fee() function async because of reqwests default features. This could be an issue for some crate consumers.

Task

Create a feature for festivus that allows the library user to use the library synchronously.

Requirements

1 .When a user adds the crate to their project they should be able to flag "blocking" as a feature.

festivus = { git = "https://github.com/BlockSpaces/festivus", features = ["blocking"] }

2. Test cases for both async and sync code

[ColinRhys]

• I would do this second
• Create feature flag "blocking" in cargo.toml file in [features] section
• Modify the "calculate_fee" function to support asynchronous and synchronous based on the feature flag
◦ #[cfg(feature = "blocking")] vs #[cfg(not(feature = "blocking"))]
• Create tests for both asynchronous and synchronous
◦ ensure run based on the feature flag
• Create documentation of new feature