feat: expose Big Segments via public Client and Config APIs

[beekld]

Expose Big Segments to customer code

Makes the server-side Big Segments feature reachable from customer code: a public config method to wire a store, and a status provider on the client for querying store health and subscribing to changes. Builds on SDK-2366-2.

What's in

• Promotes BigSegmentsBuilder to the public include tree and adds ConfigBuilder::BigSegments(BigSegmentsBuilder), surfaced as an optional Config::BigSegments().
• New public IBigSegmentStoreStatusProvider and BigSegmentStoreStatus (IsAvailable()/IsStale()), plus Client::BigSegmentStoreStatus().
• ClientImpl constructs the BigSegmentStoreWrapper from config, hands it to the evaluator, and starts its background poll. An internal adapter exposes the wrapper's status broadcaster as the public provider.
• BigSegmentsBuilder::Build() now returns a new error code when the store is null.

Design notes

• The status surface mirrors DataSourceStatus: the accessor is Status(), and listeners use OnBigSegmentStoreStatusChange(handler) -> IConnection.
• When no store is configured, the provider reports the store as unavailable and registered listeners never fire.

Testing

Unit tests added for the new behavior.