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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