[FME-12059] SDK_UPDATE with metadata

[ZamoraEmmanuel]

JavaScript commons library

What did you accomplish?

How do we test the changes introduced in this PR?

Extra Notes

[EmilianoSanchez]

I would remove this type in order to simplify the Public API. This type is not used anywhere internally, and from the perspective of the TypeScript user, it doesn't seem very useful:

client.on(client.Event.SDK_UPDATE, (metadata) => {
  if (metadata.type === 'FLAGS_UPDATED' as SplitIO.SdkUpdateMetadataKeys['FLAGS_UPDATE']) {
    doSomethingForFlagsUpdated
  }
});

vs

client.on(client.Event.SDK_UPDATE, (metadata) => {
  if (metadata.type === 'FLAGS_UPDATED') {
    doSomethingForFlagsUpdated
  }
});

I would remove it for now and reconsider adding it or something else in the future.
Besides, we are using union types for similar string enum types (example SDKMode, StorageType, etc).

[ZamoraEmmanuel]

what do you think about replacing it with something like

  /**
   * Metadata for the update event emitted when the SDK cache is updated with new data for flags or segments.
   */
  type SdkUpdateMetadata = {
    /**
     * The type of update event.
     */
    type: SdkUpdateMetadataKey
    /**
     * The names of the flags or segments that were updated.
     */
    names: string[]
  }

  /**
   * Metadata keys for SDK update events.
   * Use the string literals directly: 'FLAGS_UPDATE' or 'SEGMENTS_UPDATE'
   */
  type SdkUpdateMetadataKey = 'FLAGS_UPDATE' | 'SEGMENTS_UPDATE';