chore: Provide selector for use as basis on FDv2 data sources

[keelerm84]

Note

Introduce SelectorStore and propagate selectors/basis through FDv2 initializers/synchronizers, updating polling/streaming to use it and adjusting tests/utilities.

• Core API (protocols):
  • Add SelectorStore with selector(); update Initializer.fetch and Synchronizer.sync to accept SelectorStore.
• Polling (ldclient/impl/datasourcev2/polling.py):
  • fetch/sync now take SelectorStore and pass ss.selector() to requester.
  • _poll builds Basis using headers and ChangeSet.selector.
• Streaming (ldclient/impl/datasourcev2/streaming.py):
  • SseClientBuilder and create_sse_client accept SelectorStore; supply query_params with basis from selector.
  • sync now takes SelectorStore and uses new builder signature.
• FDv2 orchestration (ldclient/impl/datasystem/fdv2.py):
  • Pass store (as SelectorStore) to initializers and synchronizers; consume updates accordingly.
• Testing & mocks:
  • Update tests to pass MockSelectorStore with Selector.no_selector(); add MockSelectorStore.
  • Test data source V2: adapt to new method signatures; replace close() with stop().

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