EvaluationDetails should be a string, SDK should export list of standard Reasons.

[toddbaert]

The EvaluationDetails.reason should be a string, and the SDK should export a default set of reasons, which include the current enum value as well as STATIC and CACHED.

See: https://openfeature.dev/specification/types/#resolution-details
See also https://github.com/open-feature/spec/releases/tag/v0.5.0 and https://github.com/open-feature/spec/releases/tag/v0.5.2)

Definition of done:

• EvaluationDetails.reason is made an optional string
• SDK exports set of standard reasons, including STATIC and CACHED, with inline documentation matching the table here.