Add flagsmith_feature_segment_overrides resource for SSOT segment override management #219
Description
Context
Raised in #212 — when segment overrides are created outside of Terraform (e.g. via the Flagsmith UI), terraform apply does not remove them because Terraform only manages resources it knows about. The current per-resource model (flagsmith_feature_state with segment_id) requires users to manually import any externally created overrides, which is error-prone.
Proposal
Add a new resource that manages the complete set of segment overrides for a given feature+environment, so that Terraform can treat the declared overrides as the single source of truth. Any overrides that exist on the server but are not declared in the resource would be removed on apply.
This would be opt-in and backwards-compatible — the existing flagsmith_feature_state resource would continue to work as-is.