Implement `ArrowArray*` helpers

[paleolimbot]

Now that we have buffer holders (struct ArrowBuffer) we can implement an owning struct ArrowArray. I envision the API something like the ArrowSchema* helpers:

• ArrowArrayInit(struct ArrowArray* array, int64_t n_buffers)
• ArrowArrayAllocateChildren(struct ArrowArray* array, int64_t n_children)
• ArrowArraySetBuffer(struct ArrowArray* array, int64_t i, struct ArrowBuffer* buffer) (moves the struct ArrowBuffer so that it lives in memory owned by array->private_data and sets array->buffers[i] = buffer->data).

[lidavidm]

Maybe Init could take the type and then it would know how many buffers to allocate? (Ditto for AllocateChildren)

[paleolimbot]

Ah that makes sense, and parallels ArrowSchemaInit() (where the caller can fall back on NANOARROW_TYPE_UNINITIALIZED if they're implementing their own logic). I think ArrowArrayAllocateChildren() has to exist on its own, but the utility of a helper like ArrowArrayInitFromSchemaView(struct ArrowArray* array, struct ArrowSchemaView* view, struct ArrowError* error) might become clear at some point (since the schema view contains all the information needed to initialize all the buffers for all the recursive children).