feat(superposition): Introduce targeting key support for Superposition experiment traffic splitting

[aniketburman014]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

Targeting key support for Superposition experiments

Superposition uses a targeting key to split traffic across experiment variants. Previously, the targeting key was always passed as None, so experiment-based config resolution wasn't functional.

Added targeting_key as a type parameter in the config! macro so each config declares which ID type is used for traffic splitting. The macro-generated method accepts the ID directly and converts it to a String before passing it to Superposition's OpenFeature evaluation context.

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

1. Set up an experiment in Superposition on requires_cvv with two variants (e.g., 50/50 split)
2. Create multiple customers with different customer IDs
3. Create a payment method for each customer
4. Get payment token for each customer
5. Check logs — different customer IDs should resolve to different requires_cvv values
6. Repeat a payment for the same customer — should get the same variant every time

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible

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Changed Files

File	Status
crates/router/src/core/payment_methods/cards.rs	84% smaller
crates/router/src/core/configs.rs	72% smaller
crates/router/src/core/configs/dimension_config.rs	49% smaller
crates/external_services/src/superposition.rs	38% smaller
Cargo.lock	Unsupported file format
crates/external_services/Cargo.toml	Unsupported file format
crates/router/src/core/payments/helpers.rs	0% smaller