[Python][FlightRPC] FlightServer get_schema raises an error if it returns a Schema instance, requires a SchemaResult

When a user is designing a get_schema() method for a FlightServer, the expected usage is that it should return a "schema" instance, as it says on the docs. However, the code actually raises an error if a schema is returned, and requires you to return an instance of SchemaResult.

The bug is caused by lines 2059-262 of pyarrow._flight, in the method _get_schema:

```
if not isinstance(result, SchemaResult):
raise TypeError("FlightServerBase.get_schema_info must return "
"a SchemaResult instance, but got {}".format(
type(result)))
```

I think this could be corrected by changing SchemaResult to Schema, and parsing the Schema as a SchemaResult internally.

### Component(s)

FlightRPC