feat: add renaming mechanism for EnvVarProvider

[MatteoJoliveau]

EnvVarProvider now supports injecting a custom renaming strategy, which allows users to customise how environment variables are selected by transforming the requested flag key.

This PR

• adds the Rename trait to EnvVarProvider, which allows users to inject custom strategies for transforming flag keys into the corresponding environment variable name.

How to test

Unit tests have been added. I tried to add a case to the Cucumber world, but I'm not familiar with that system, and I had issues injecting a generic R into the World.

[gemini-code-assist]

Summary of Changes

Hello @MatteoJoliveau, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the EnvVarProvider by introducing a flexible mechanism for transforming feature flag keys into environment variable names. By allowing the injection of custom renaming strategies via a new Rename trait, the provider can now adapt to diverse naming conventions without requiring direct modifications to its core logic. This change improves the provider's adaptability and extensibility for various use cases.

Highlights

• Custom Renaming Strategy: The EnvVarProvider now supports injecting a custom renaming strategy, allowing users to define how feature flag keys are transformed into environment variable names. This provides greater flexibility in matching different naming conventions.
• New Rename Trait: A new Rename trait has been introduced, defining a rename method that takes a flag key and returns a Cow<'a, str>. This trait is implemented by a default NoopRename (which performs no transformation) and also for closures, enabling easy custom renaming logic.
• Generic EnvVarProvider: The EnvVarProvider has been made generic over the Rename trait, allowing it to accept and utilize any custom renaming strategy. The internal evaluate_environment_variable function now uses this injected strategy to determine the correct environment variable name.

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[gemini-code-assist]

Code Review

This pull request introduces a flexible renaming mechanism for the EnvVarProvider by using a generic Rename trait. The implementation is well-designed, particularly the use of Cow for efficiency and the blanket implementation for closures, which makes the API very ergonomic. My review includes a few suggestions to improve code idiom and test clarity. Overall, this is a great addition.

[gemini-code-assist]

In Rust, the return keyword is typically omitted for the final expression of a function, as the last expression is implicitly returned. Removing it makes the code more idiomatic and aligns with common Rust style.

Suggested change

	return evaluate_environment_variable(&self.rename, flag_key, evaluation_context);
	evaluate_environment_variable(&self.rename, flag_key, evaluation_context)

[MatteoJoliveau]

While I agree with this suggestion (and can add a commit fixing this if maintainers agree), I have to point out that this was not a change I introduced 😅

[gemini-code-assist]

In Rust, the return keyword is typically omitted for the final expression of a function. It's more idiomatic to rely on the implicit return.

Suggested change

	return evaluate_environment_variable(&self.rename, flag_key, evaluation_context);
	evaluate_environment_variable(&self.rename, flag_key, evaluation_context)

[gemini-code-assist]

In Rust, the return keyword is typically omitted for the final expression of a function. It's more idiomatic to rely on the implicit return.

Suggested change

	return evaluate_environment_variable(&self.rename, flag_key, evaluation_context);
	evaluate_environment_variable(&self.rename, flag_key, evaluation_context)