Allow non-nullable schemas to be created

We commonly use a combination of columns that are non-nullable and nullable. The current default of 'always nullable' from e.g. line 179 here is a little strict.

https://github.com/apache/arrow-nanoarrow/blob/75ef830c80b0edcbd3604eb013497e1792dfca69/src/nanoarrow/common/schema.c#L175-L185

Obviously, we can add a local variant with an added bool which calls this and then, as needed, follows up with

schema->flags &= ~ARROW_FLAG_NULLABLE;

But can you see a way to more cleanly allow this? There are a few spots that condition / undo but they seem like lower-level entry points. If I am missing something really obvious don't be shy and call me out :grinning:

Start in src/nanoarrow/common/schema.c around lines 175-185 and trace the schema creation entry points that impose the always-nullable default. Determine a clean API-level way to create non-nullable schemas without breaking the existing default behavior; done means mixed nullable and non-nullable column schemas can be created without local flag manipulation.

Written by the indexing model from the issue text.