SQL function: `FirstValue` and `LastValue`

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## first_value
`first_value(expr[, isIgnoreNull]) `
- Returns the first value of expr for a group of rows. If isIgnoreNull is true, returns only non-null values.

### Examples:
```SQL

> SELECT first_value(col) FROM VALUES (10), (5), (20) AS tab(col);
10
> SELECT first_value(col) FROM VALUES (NULL), (5), (20) AS tab(col);
NULL
> SELECT first_value(col, true) FROM VALUES (NULL), (5), (20) AS tab(col);
5
```

### Note:
The function is non-deterministic because its results depends on the order of the rows which may be non-deterministic after a shuffle.

## last_value
l`ast_value(expr[, isIgnoreNull])`
- Returns the last value of expr for a group of rows. If isIgnoreNull is true, returns only non-null values

### Examples:
```SQL
> SELECT last_value(col) FROM VALUES (10), (5), (20) AS tab(col);
20
> SELECT last_value(col) FROM VALUES (10), (5), (NULL) AS tab(col);
NULL
> SELECT last_value(col, true) FROM VALUES (10), (5), (NULL) AS tab(col);
5
```

### Note:

The function is non-deterministic because its results depends on the order of the rows which may be non-deterministic after a shuffle.

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目前HybridSE的在线数据是默认倒序的。在获取firstvalue和lastvalue需要特别关注这个问题
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**Additional context**
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The issue is about implementing SQL window functions `first_value` and `last_value` in OpenMLDB, with an optional null-ignoring flag. Look for existing window function implementations in the codebase, likely in SQL parser and execution engine. Understand how online data ordering works (default reverse order). Start by examining the function registration and evaluation logic for aggregate/window functions. Write tests to verify behavior with and without the isIgnoreNull parameter.

Written by the indexing model from the issue text.