fix: HashJoin panic with dictionary-encoded columns in multi-key joins (apache#20441)

## Which issue does this PR close?

- Closes apache#20437


## Rationale for this change
`flatten_dictionary_array` returned only the unique values rather then
the full expanded array when being called on a `DictionaryArray`. When
building a `StructArray` this caused a length mismatch panic.


## What changes are included in this PR?
Replaced `array.values()` with `arrow::compute::cast(array, value_type)`
in `flatten_dictionary_array`, which properly expands the dictionary
into a full length array matching the row count.

## Are these changes tested?

Yes, both a new unit test aswell as a regression test were added.

## Are there any user-facing changes?

Nope

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: 2 people

datafusion/physical-plan/src/joins/hash_join/inlist_builder.rs

@@ -20,6 +20,7 @@
 use std::sync::Arc;
 
 use arrow::array::{ArrayRef, StructArray};
+use arrow::compute::cast;
 use arrow::datatypes::{Field, FieldRef, Fields};
 use arrow_schema::DataType;
 use datafusion_common::Result;