feat: Add the FDv2 streaming synchronizer to the client SDK

[beekld]

Summary

Adds the client SDK's FDv2 streaming synchronizer.

Implemented here:

• The SSE stream reader, driving FDv2ProtocolHandler into source results.
• Per-connect request construction, goodbye and ping handling, and response-header parsing.
• SSE error classification into Interrupted and TerminalError.

Differences from the Android and Flutter clients:

• Ping: this SDK drops a ping that arrives while an answering poll is in flight, where Flutter coalesces overlapping pings into one follow-up poll and Android blocks the stream thread on a synchronous poll.

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Note

Overview
Adds FDv2StreamingSynchronizer to the client SDK so FDv2 flag updates can stay current over a long-lived SSE connection, exposed through the same IFDv2Synchronizer::Next() pull interface as polling.

The synchronizer builds /sdk/stream/eval requests (GET with base64 context in the path or POST with JSON body), applies basis from the latest selector on each connect, and drives FDv2ProtocolHandler to emit ChangeSet, Goodbye, Interrupted, and TerminalError results. ping events trigger a follow-up poll (overlapping pings are dropped while a poll is in flight). Response headers and goodbye payloads surface environment id and FDv1 fallback directives, with stream-level TTL overriding headers.

Also adds a ReadFDv2ResponseHeaders overload for Boost.Beast response headers, links launchdarkly::sse in client SDK tests, adds broad unit coverage via a test peer, tweaks server-sdk test mocks/formatting, and makes SSE read timeout log messages omit the duration when no timeout value is present.

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