Index segment references to dependent flags

[khvn26]

Add a dependency index model:

SegmentFlagReference(segment, prerequisite_feature, rule_path)

Acceptance criteria

• Creating a segment with $.flags conditions creates one index row per referenced feature.
• Conditions nested in ALL/ANY/NONE rules at any depth are indexed, with a rule_path that points to the originating condition.
• Editing a segment to add, change or remove a $.flags condition updates the index to match.
• Deleting or soft-deleting a segment removes its index rows.
• Publishing a new segment version updates the index row.

  Moved to https://github.com/Flagsmith/flagsmith-private/issues/296.
  -- Evandro

• A condition referencing a non-existent feature name does not break indexing.

  Decided to reject referencing unknown features, as that would lead to a stale index if the prerequisite feature is created at a later moment. Therefore, a feature needs to exist prior to being made a prerequisite.
  -- Evandro

• Indexing adds no measurable latency to segment writes on a project with a realistic segment count.
• Attempting to persist a condition that would close a cycle against live versions any of the project's environment via any amount of nodes raises a validation error that would translate to a user-facing 400 with code: circular_dependency and a path naming each feature + segment.

[emyller]

• A condition referencing a non-existent feature name does not break indexing.

Decided to reject referencing unknown features, as that would lead to a stale index if the prerequisite feature is created at a later moment. Therefore, a feature needs to exist prior to being made a prerequisite.

Also updated in the issue description.

[emyller]

• Publishing a new segment version updates the index row.

Moved to https://github.com/Flagsmith/flagsmith-private/issues/296, which is blocked until #8537 is merged.