chore: commonizes several FDv2 related types in Common

[tanderson-ld]

This PR moves several types from java server to java common for re-use in Android SDK. ChangeSet has been made generic on the type of the data. There should be no behavioral changes due to this PR.

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Note

Low Risk
Low risk: this PR only adds new shared types/enums (no runtime logic changes) but may require downstream modules to update imports/types when adopting the new ChangeSet<T> API.

Overview
Commonizes FDv2 model types in lib/shared/common. Adds a new com.launchdarkly.sdk.fdv2 package with shared public types for FDv2 data sources.

Introduces a generic ChangeSet<T> (with ChangeSetType, Selector, and persistence/environment metadata) plus enums SourceResultType and SourceSignal, and documents intended usage via package-info.java.

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feat: commonizes several FDv2 related types
END_COMMIT_OVERRIDE

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[tanderson-ld]

For reviewers: Renamed this from FDv2SourceResult.State to SourceSignal to reduce similarity to a very similar DataSourceStatusProvider enum that already exists that talks about data source state.

  /**
   * An enumeration of possible values for {@link DataSourceStatusProvider.Status#getState()}.
   */
  public enum State {