chore(.github): add to bigtable owners
# Default owner for all directories not owned by others
* @googleapis/yoshi-go-admins
-/bigtable/ @crwilcox @tritone @googleapis/yoshi-go-admins
+/bigtable/ @crwilcox @tritone @kolea2 @googleapis/yoshi-go-admins
/bigquery/ @googleapis/api-bigquery @googleapis/yoshi-go-admins
/datastore/ @crwilcox @tritone @googleapis/yoshi-go-admins
/firestore/ @crwilcox @tritone @googleapis/yoshi-go-admins
