feat: Add a start wait timeout for initialization

[kinyoklion]

Adds a start_wait parameter to LaunchDarklyProvider, defaulting to the LaunchDarkly SDK's five seconds.

• The value is passed to LDClient(config, start_wait) and bounds the whole of initialization: with a positive value initialize completes with the outcome the constructor already waited for, rather than waiting again, so a 5 second start wait cannot become a 10 second wait for set_provider_and_wait.
• Initialization fails when the client did not become ready in time; the provider keeps reporting status afterward, so a later connection still makes it ready.
• Zero does not block the constructor at all, and initialize then waits without a deadline.

Implementation details

# With a start wait the client constructor has already waited, so the outcome is whatever it is now.
if self.__start_wait <= 0:
    ready_event.wait()

Testing: make test, make lint. Tests cover the default matching the SDK default and initialization failing without waiting a second time.

Related spec change: https://github.com/launchdarkly/sdk-specs/pull/257

Link to Devin session: https://app.devin.ai/sessions/0c452d209ec54b068ba120b4c92b8f6c
Open in Devin Desktop: https://app.devin.ai/desktop/session/0c452d209ec54b068ba120b4c92b8f6c?variant=devin
Requested by: @kinyoklion

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