chore(java-spanner): add code owners for java-spanner

Author: chingor13

.github/CODEOWNERS

@@ -9,3 +9,4 @@
 /java-vertexai/   @googleapis/vertexai-team @googleapis/cloud-sdk-java-team
 /java-bigquerystorage/ @googleapis/bigquery-team @googleapis/cloud-sdk-java-team
 /java-bigquery/ @googleapis/bigquery-team @googleapis/cloud-sdk-java-team
+/java-spanner/ @googleapis/spanner-team @googleapis/cloud-sdk-java-team