Hi

How do you go about implementing an extension type through the C API for Arrow? 

Creating a record batch in python like the example in pyarrow tests [here](https://github.com/apache/arrow/blob/2a5f92455ec4f9788ee96fa209b38d76bd927196/python/pyarrow/tests/test_extension_type.py#L375), and reading the resulting schema using the C API, it correctly reads it as an array of the underlying storage type. The schema along with the metadata can be printed as expected. 
```
ext: int64
-- metadata --
ARROW:extension:metadata: freq=D
ARROW:extension:name: test.period
```

However, trying to access the metadata for the schema (obtained with `garrow_schema_get_metadata`) indicates its size to be 0, which indicates that metadata for the schema isn't the same as that for a field. Is it possible using the existing API to read/write the metadata for a field?


Thank you,
Ishan