SQL: support DATE literal and TIMESTAMP literal

**Is your feature request related to a problem? Please describe.**

Now, we have numeric literals and string literals in OpenMLDB. Users are expecting to use DATE literal and TIMESTAMP literal as described in Standard SQL.

In this issue, we are going to support DATE literal and TIMESTAMP literals
```sql
::= DATE
::=

::= TIMESTAMP
::=

::=
::=
::=

::= [ ]
::=
::=

```

```sql
SELECT TIMESTAMP "2021-05-20 10:11:12+08", DATE "2021-05-20";
```

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**Additional context**
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The issue involves extending the SQL parser to recognize DATE and TIMESTAMP literals. Look at the existing literal parsing code in the OpenMLDB codebase, likely in the SQL parser module. Understand the grammar rules provided and implement support for the new literal types. Test with SQL statements like SELECT TIMESTAMP "2021-05-20 10:11:12+08", DATE "2021-05-20"; to ensure they are correctly parsed and evaluated.

Written by the indexing model from the issue text.