Expose NullDataSourceBuilder from crate root

This may seem a bit strange at first, but it's extremely useful when
a persistent `DataStore` is filled externally to the LaunchDarkly SDK.

In our particular use case, we implement a `PersistentDataStore` using
an internal blend of Kafka and RocksDB to allow us to isolate the
outbound calls towards LD to a single service.

An alternative to this is allowing `offline` mode to continue reading
from the `DataStore`, but this is a breaking change.

Author: w4

launchdarkly-server-sdk/src/lib.rs

@@ -29,7 +29,8 @@ pub use client::Client;
 pub use client::{BuildError, StartError};
 pub use config::{ApplicationInfo, BuildError as ConfigBuildError, Config, ConfigBuilder};
 pub use data_source_builders::{
-    BuildError as DataSourceBuildError, PollingDataSourceBuilder, StreamingDataSourceBuilder,
+    BuildError as DataSourceBuildError, NullDataSourceBuilder, PollingDataSourceBuilder,
+    StreamingDataSourceBuilder,
 };
 pub use evaluation::{FlagDetail, FlagDetailConfig};
 pub use events::event::MigrationOpEvent;