feat!: Add a start wait time option for initialization

[kinyoklion]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Fixes #58.

Describe the solution you've provided

Adds Provider(String sdkKey, LDConfig config, Duration startWait) so an application can bound, or deliberately opt out of, waiting for initialization, matching the OpenFeature provider spec (OFP) requirements for start wait behavior:

• a positive Duration is applied by the LaunchDarkly client constructor, and initialize then reports whether the client became ready without waiting a second time
• Duration.ZERO waits nowhere; initialize fails unless the client is already ready, and the application learns when it becomes usable from provider events
• null requests indefinite waiting: nothing is waited for during construction and initialize blocks until the data source becomes valid or fails permanently

The existing constructors keep honouring LDConfig.Builder.startWait(...) and no longer wait again in initialize, so a configured start wait is the only bound on initialization instead of being followed by an unbounded wait. The README feature matrix row and initialization section describe the new behavior.

Implementation details

initialize still registers data source and flag change listeners and evaluates the current data source status. Only the indefinite mode leaves the completion future pending; every other mode completes it with the outcome that is already known, so initialize returns promptly. Status reporting after a failed initialization is unchanged.

Describe alternatives you've considered

Keeping the existing indefinite wait and adding a timeout parameter interpreted only inside initialize would double-wait when LDConfig.startWait is also set, and would not match OFP, which treats the start wait as a bound over construction plus initialization.

Additional context

Sibling changes for the same OFP requirement: launchdarkly/openfeature-python-server#64, launchdarkly/openfeature-ruby-server#36, launchdarkly/openfeature-dotnet-server#71, and launchdarkly/js-core#2024.

Verified with ./gradlew test (all tests pass, including new coverage for positive, zero, and indefinite start wait).

Link to Devin session: https://app.devin.ai/sessions/1fd3fcbfe79f482e8b58be29e8df2ffb
Open in Devin Desktop: https://app.devin.ai/desktop/session/1fd3fcbfe79f482e8b58be29e8df2ffb?variant=devin
Requested by: @kinyoklion

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