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ARROW-10679: [Rust] [DataFusion] Implement CASE WHEN physical expression

This PR implements the physical expression for the SQL CASE expression.

The CASE expression is similar to a series of nested if/else and there are two forms that can be used.

The first form consists of a series of boolean "when" expressions with corresponding "then" expressions and an optional "else" expression.

```sql
CASE WHEN condition THEN result
     [WHEN ...]
     [ELSE result]
END
```

The second form uses a base expression and then a series of "when" clauses that match on a literal value.

```sql
CASE expression
    WHEN value THEN result
    [WHEN ...]
    [ELSE result]
END
```

The implementation is quite simple/naive and I have not attempted to optimize this yet. I would prefer to get this plumbed through and tested with TPC-H before optimizing.

Closes #8740 from andygrove/case-when

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>