feat!: Add a start wait time option for initialization

[kinyoklion]

Aligns initialization with the OpenFeature provider spec (OFP) start wait requirements, so waiting for the client is bounded instead of unbounded.

• New Provider(Configuration config, TimeSpan? startWait) constructor
• A positive start wait time bounds construction, and InitializeAsync then reports the outcome without waiting again
• TimeSpan.Zero waits nowhere; readiness is reported through provider events
• null waits indefinitely for the data source to become valid or fail permanently
• Breaking: InitializeAsync no longer waits indefinitely after a configured StartWaitTime elapses

Implementation details

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

Describe the solution you've provided

InitializeAsync still registers the flag and data source listeners and checks the current data source status. Only the indefinite mode leaves the initialization task pending; every other mode completes it with the outcome that is known at that point, so initialization does not wait a second time after the LaunchDarkly client constructor has already waited.

The README feature matrix row for initialization and the asynchronous initialization section were updated to describe the three modes.

Describe alternatives you've considered

Interpreting a timeout only inside InitializeAsync would wait twice when StartWaitTime is also configured, and OFP treats the start wait time as a bound over construction plus initialization.

Additional context

Sibling changes for the same OFP requirement: launchdarkly/openfeature-java-server#66, launchdarkly/openfeature-python-server#64, launchdarkly/openfeature-ruby-server#36.

Out of scope, per the audit request: the dependency injection package and the client-side paradigm.

Verified with dotnet test -f net8.0 (71 tests pass).

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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