Move SnapshotConfig into its own module (#18331)

Also move ArchiveFormat to snapshot_utils, and do not
reexport SnapshotVersion.

runtime/src/accounts_background_service.rs

@@ -4,7 +4,8 @@
 
 use crate::{
     bank::{Bank, BankSlotDelta, DropCallback},
-    bank_forks::{BankForks, SnapshotConfig},
+    bank_forks::BankForks,
+    snapshot_config::SnapshotConfig,
     snapshot_package::AccountsPackageSender,
     snapshot_utils,
 };