Field metadata for dictionary-encoded extension types is lost

**Describe the bug**

The representation of Dictionary in the data types enum seems to exclude field metadata, so extension types are dropped when they go through arrow-rs structures:

https://github.com/apache/arrow-rs/blob/a7f3ba8f3a748243af1575bce8d50dfc6a81ab73/arrow-schema/src/datatype.rs#L359

The definition of RunEndEncoded and others seem to use a `FieldRef` and I'm wondering if it was a deliberate choice not to do this or whether it's just never come up.

**To Reproduce**

I used arro3 to reproduce:

```python
import arro3.core as a3
import geoarrow.pyarrow as ga
import nanoarrow as na
import pyarrow as pa

c_schema = na.c_schema(pa.dictionary(pa.int32(), ga.wkb()))

c_schema.metadata is None
#> True
c_schema.dictionary.metadata
#>
#> - b'ARROW:extension:name': b'geoarrow.wkb'
#> - b'ARROW:extension:metadata': b'{}'

c_schema2 = na.c_schema(a3.DataType.dictionary(pa.int32(), ga.wkb()))
c_schema2.metadata is None
#> True
c_schema2.dictionary.metadata is None
#> True
```

**Expected behavior**

I would have expected the metadata to roundtrip through the arrow-rs data type representation

**Additional context**

Occasionally Parquet readers will return dictionary-encoded arrays on read whose representation is not entirely in control of the user.