feat: Add FDv2 polling sources to the client

[beekld]

Summary

Adds the FDv2 polling sources -- a one-shot polling initializer and an interval-based polling synchronizer -- over a shared request builder and response handler that feed the response through the protocol handler and the changeset translation.

• The evaluation context travels base64url-encoded in the request path (default) or in a POST body. The FDv1 REPORT option is not carried forward.
• The synchronizer sends the selector's state as the basis query parameter for deltas, with a poll-interval floor to avoid hammering the service.
• The environment id and any FDv1 fallback directive are read from the response headers. A directive carried on a goodbye message takes precedence.

Design note: this client sends no conditional-request validator (If-None-Match/ETag), unlike the Flutter and Android clients, which track a per-context ETag and act on real 304 responses. The spec lists omitting If-None-Match entirely as an accepted way to avoid the cross-context ETag hazard, and since every SDK already sends the basis selector for deltas, dropping the ETag costs little here. Because no If-None-Match is ever sent, a 304 should not occur, so the 304 -> none handling is defensive.

Internal only. The orchestrator that runs these arrives in a later change.

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Note

Overview
Adds FDv2 HTTP polling to the C++ client SDK: shared plumbing to build /sdk/poll/eval requests and turn poll responses into FDv2SourceResult via the existing protocol handler and changeset translation.

FDv2PollingInitializer performs a one-shot basis load; FDv2PollingSynchronizer polls on an interval (clamped to the SDK minimum), sends selector state as basis, and supports cancel/shutdown via Close(). Context can travel as base64url in the GET path or JSON in a POST body (FDv2RequestConfig). The client does not send If-None-Match; 304 is still mapped defensively to a none changeset. Response headers supply environment id and optional FDv1 fallback (body goodbye TTL wins over header).

AsioRequester is refactored so redirect/async paths capture the SSL context explicitly instead of this, avoiding lifetime issues in nested callbacks.

Unit tests cover request shaping, response/error/fallback behavior, and synchronizer timing/close semantics. Wiring into the public data-source orchestrator is left for a follow-up.

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