feat: implement evaluation details and re-design evaluation API

[sheepduke]

This PR

• Implement evaluation details get_xx_details for the client.
• Re-designed the API to return Result<T, EvaluationError> and Result<EvaluationDetails<T>, EvaluationError> for get_xx_value and get_xx_details respectively.
• Remove default_value parameter from the evaluation functions.
• Add tests for all the spec requirements already supported.
• Enhance README to reflect current change.
• Add Apache 2 license file.
• Eliminates all the warnings.

Related Issues

Notes

Result<T, E> is a powerful tool to build abstractions in Rust. By utilizing this instead of putting everything into one huge struct, we gain the following merits:

• The spec indicates that default_value should always be returned when abnormal execution. Meaning its value is passed into each function and then (conditionally) returned back. It does not make much sense because caller could simply write client.get_int_value(..).unwrap_or(123) to provide 123 as a default value. There is no need to put it into the function parameter and in the return value.

• The user could more easily decide what to do when the evaluation fails. Result provides many helper functions like map, unwrap_or etc.

• Success cases and failure cases are strictly distinguished with 2 branches.

• The abstractions and APIs become cleaner.

Follow-up Tasks

1. Release 0.1.0.
2. Implement hooks and events.
3. Address other issues.

How to test

UT.

[beeme1mr]

This pr is massive 😅. I'll take a look tomorrow.

[sheepduke]

This pr is massive 😅. I'll take a look tomorrow.

Totally understood. Thanks for helping review it! :-D

I wanted to make it small, but the changes to the core evaluation abstractions brought too many changes...