Implement `unique` function

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Part of https://github.com/apache/arrow-datafusion/issues/7289

**Describe the solution you'd like**
`unique` function removes duplicate values (include NULLs) from `Array`. Does preserve the original order.

Example (PyArrow):
```
>>> pc.unique(pa.array([1, 2, 1, 4, 5, 2, 4, 1]))

[
1,
2,
4,
5
]
```

with nulls:

```
>>> pc.unique(pa.array([1, 2, 1, 4, None, 5, 2, None, 4, 1]))

[
1,
2,
4,
null,
5
]
```

**Describe alternatives you've considered**

**Additional context**
Documentation: https://arrow.apache.org/docs/cpp/compute.html

Start with the linked Arrow compute documentation and the examples in this issue, then locate the Rust array compute entry points for implementing functions. Done means a unique operation preserves first-seen order, removes duplicate values, and includes at most one NULL; no specific source file or test is named in the issue.

Written by the indexing model from the issue text.