[BUG] Unified query API fails with date types

What is the bug?
Unified query API fails when date type is involved in PPL, but query runs fine in OpenSearch

How can one reproduce the bug?
Run the following class:

package org.opensearch.sql.api.examples;

import java.sql.Date;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import org.apache.calcite.DataContext;
import org.apache.calcite.config.CalciteConnectionConfig;
import org.apache.calcite.linq4j.Enumerable;
import org.apache.calcite.linq4j.Linq4j;
import org.apache.calcite.rel.type.RelDataType;
import org.apache.calcite.rel.type.RelDataTypeFactory;
import org.apache.calcite.schema.ScannableTable;
import org.apache.calcite.schema.Schema;
import org.apache.calcite.schema.Statistic;
import org.apache.calcite.schema.Statistics;
import org.apache.calcite.schema.Table;
import org.apache.calcite.schema.impl.AbstractSchema;
import org.apache.calcite.sql.SqlCall;
import org.apache.calcite.sql.SqlNode;
import org.apache.calcite.sql.type.SqlTypeName;
import org.opensearch.sql.api.UnifiedQueryContext;
import org.opensearch.sql.api.UnifiedQueryPlanner;
import org.opensearch.sql.api.compiler.UnifiedQueryCompiler;
import org.opensearch.sql.executor.QueryType;

/** MWE: DATE() function comparison fails - returns string instead of int (days since epoch). */
public class DateComparisonBugExample {

  public static void main(String[] args) throws Exception {
    Map<String, SqlTypeName> schema = new LinkedHashMap<>();
    schema.put("id", SqlTypeName.INTEGER);
    schema.put("date_hired", SqlTypeName.DATE);

    List<Object[]> rows = List.of(new Object[] {1, Date.valueOf("2020-03-15")}, new Object[] {2, Date.valueOf("2020-06-15")});

    AbstractSchema testSchema =
        new AbstractSchema() {
          @Override
          protected Map<String, Table> getTableMap() {
            return Map.of("employees", new SimpleTable(schema, rows));
          }
        };

    try (UnifiedQueryContext context =
        UnifiedQueryContext.builder()
            .language(QueryType.PPL)
            .catalog("test", testSchema)
            .defaultNamespace("test")
            .build()) {

      UnifiedQueryPlanner planner = new UnifiedQueryPlanner(context);
      UnifiedQueryCompiler compiler = new UnifiedQueryCompiler(context);

      String query = "source=employees | where date_hired > DATE('2020-06-01')";
      PreparedStatement stmt = compiler.compile(planner.plan(query));
      ResultSet rs = stmt.executeQuery();

      while (rs.next()) {
        System.out.println("id=" + rs.getInt(1) + ", date_hired=" + rs.getDate(2));
      }
    }
  }

  private static class SimpleTable implements ScannableTable {
    private final Map<String, SqlTypeName> schema;
    private final List<Object[]> rows;

    SimpleTable(Map<String, SqlTypeName> schema, List<Object[]> rows) {
      this.schema = schema;
      this.rows = rows;
    }

    @Override
    public RelDataType getRowType(RelDataTypeFactory typeFactory) {
      RelDataTypeFactory.Builder builder = typeFactory.builder();
      schema.forEach((name, type) -> builder.add(name, typeFactory.createSqlType(type)));
      return builder.build();
    }

    @Override
    public Enumerable<Object[]> scan(DataContext root) {
      return Linq4j.asEnumerable(rows);
    }

    @Override
    public Statistic getStatistic() {
      return Statistics.UNKNOWN;
    }

    @Override
    public Schema.TableType getJdbcTableType() {
      return Schema.TableType.TABLE;
    }

    @Override
    public boolean isRolledUp(String column) {
      return false;
    }

    @Override
    public boolean rolledUpColumnValidInsideAgg(
        String column, SqlCall call, SqlNode parent, CalciteConnectionConfig config) {
      return false;
    }
  }
}

What is the expected behavior?

The PPL query executed in the class should run fine, and return the second record in the table.

This expected behavior can be reproduced in OpenSearch:

PUT /employees
{
  "mappings": {
    "properties": {
      "id": { "type": "integer" },
      "date_hired": { "type": "date", "format": "yyyy-MM-dd" }
    }
  }
}

POST /employees/_bulk
{"index": {}}
{"id": 1, "date_hired": "2020-03-15"}
{"index": {}}
{"id": 2, "date_hired": "2020-06-15"}


POST /_plugins/_ppl
{
  "query": "source=employees | where date_hired > DATE('2020-06-01')"
}