Schema DSL for testing

Arrow C++ includes factories for constructing schemas, types, fields, and metadata which allow construction of even deeply nested structures to be expressive:

```c++
schema({
field("some_col", int32(), key_value_metadata({
{"some_key_field", "some_value_field"},
})),
}, key_value_metadata({{"some_key", "some_value"}})),
```

It should be straightforward to write equivalent factories which build a `nanoarrow::UniqueSchema`.

No files or tests are named. Start by locating the existing Arrow C++ factories for schemas, types, fields, and metadata, then trace the nanoarrow::UniqueSchema entry points. Done means equivalent factories can express deeply nested schemas and metadata as concisely as the shown example.

Written by the indexing model from the issue text.