feat(CassandraClient, CassandraSource): Add comments to clarify primary key handling and projection logic

Author: darkmatter18

connector/src/main/java/com/datastax/oss/cdc/CassandraClient.java

@@ -167,6 +167,7 @@ public PreparedStatement prepareSelect(String keyspaceName, String tableName,
                                            CqlIdentifier[] projection,
                                            CqlIdentifier[] pk,
                                            int pkLength) {
+        // select columns according to projection array length
         Select query = selectFrom(keyspaceName, tableName)
                 .columns(projection.length != 0 ? projection : pk);
         for (int i = 0; i < pkLength; i++)

connector/src/main/java/com/datastax/oss/pulsar/source/CassandraSource.java

@@ -355,6 +355,7 @@ synchronized void setValueConverterAndQuery(KeyspaceMetadata ksm, TableMetadata
             List<ColumnMetadata> columns = tableMetadata.getColumns().values().stream()
                     // include primary keys in the json only output format options
                     // TODO: PERF: Infuse the key values instead of reading from DB https://github.com/datastax/cdc-apache-cassandra/issues/84
+                    // If primary key only table, then add all the columns into the value schema.
                     .filter(c -> config.isJsonOnlyOutputFormat() || isPrimaryKeyOnlyTable || !tableMetadata.getPrimaryKey().contains(c))
                     .filter(c -> !columnPattern.isPresent() || columnPattern.get().matcher(c.getName().asInternal()).matches())
                     .collect(Collectors.toList());