Refactor SegmentLoadDropHandler code (#16685)

Motivation:
- Improve code hygeiene
- Make `SegmentLoadDropHandler` easily extensible

Changes:
- Add `SegmentBootstrapper`
- Move code for bootstrapping segments already cached on disk and fetched from coordinator to
`SegmentBootstrapper`.
- No functional change
- Use separate executor service in `SegmentBootstrapper`
- Bind `SegmentBootstrapper` to `ManageLifecycle` explicitly in `CliBroker`, `CliHistorical` etc.

server/src/main/java/org/apache/druid/server/coordination/ChangeRequestsSnapshot.java

@@ -24,7 +24,6 @@
 import com.google.common.base.Preconditions;
 
 import javax.annotation.Nullable;
-
 import java.util.List;
 
 /**