explicitly initialize singleton immutablebimap

Author: diegomarquezp

src/main/resources/META-INF/native-image/com.google.cloud/google-cloud-spanner-jdbc/native-image.properties

@@ -4,6 +4,7 @@ Args = --initialize-at-build-time==com.google.cloud.spanner.IntegrationTestEnv,\
   com.google.common.collect.RegularImmutableMap,\
   com.google.common.collect.ImmutableMapEntry,\
   com.google.common.collect.ImmutableMapEntry$NonTerminalImmutableMapEntry,\
+  com.google.common.collect.SingletonImmutableBiMap,\
   com.google.cloud.spanner.Dialect,\
   com.google.cloud.spanner.Dialect$2,\
   com.google.cloud.spanner.Dialect$1,\