sampledataccl: add an abstraction for ccl testdata

Data is a representation of a sql table, used for creating test data in
various forms (sql rows, kvs, enterprise backup).

We only have one for the moment (a bank table with an id, balance, and
payload for padding row-size), but the indirection of the interface
didn't really add any complexity.

pkg/ccl/sqlccl/load.go

@@ -326,6 +326,10 @@ func writeSST(
 	kvs []engine.MVCCKeyValue,
 	ts hlc.Timestamp,
 ) error {
+	if len(kvs) == 0 {
+		return nil
+	}
+
 	filename := fmt.Sprintf("load-%d.sst", rand.Int63())
 	log.Info(ctx, "writesst ", filename)