Support creating map arrays with key metadata

[rshkv]

In iceberg-rust needs to be able to construct a MapArray with metadata on both key and value fields. The Iceberg spec requires that all fields in a table, including map keys, have a unique field identifier. And iceberg-rust tracks the field identifier as a metadata field.

MapBuilder currently only supported overriding value fields (here). We're hitting this limitation here: apache/iceberg-rust#863 (comment).

This is a request to support overriding the key field as well.

[alamb]

label_issue.py automatically added labels {'arrow'} from #7101