[FEATURE] Nested `fractional` Argument Evaluation in flagd provider

[toddbaert]

Sub-issue of open-feature/flagd#1676.

Support nested/hierarchical evaluation of arguments inside the fractional custom operation, so that arbitrary targeting rules (e.g. if, in, var, or even nested fractional) can be used within the fractional buckets. See the parent issue for full requirements and examples.

The flagd schema is being updated to support this in open-feature/flagd-schemas#207 — you can pull that schema version for validation before this feature is released.

Release strategy: this feature should be implemented and released before the high-resolution fractional bucketing feature (open-feature/flagd#1903), to avoid git conflicts. @toddbaert will coordinate the release across providers.