Support creating arrays from externally owned memory like Vec or String

[jhorstmann]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

I'd like to process data that is coming from outside arrow using arrow compute kernels without copying the data.

The FFI api does not seem to support this use case at the moment. The only way I found to create an FFI_ArrowArray was with an already existing ArrayData struct. Creating ArrayData requires at least one Buffer, and creating a Buffer from foreign memory again needs an FFI_ArrowArray.

Describe the solution you'd like

The Buffer already supports two ways of deallocating memory.

• Via arrows cache-aligned allocator
• With a callback in an FFI_ArrowArray

The second option can be generalized so that the callback can be any type using a dyn trait.

Describe alternatives you've considered

An alternative could be to make it easier to create an FFI_ArrowArray from a single memory region, but that seems to be more complex and also less general.

Additional context

This functionality could also allow zero-copy conversion from arrow2.