[Flight client] Allow disabling the creation of a child allocator

[swallez]

Describe the enhancement requested

FlightClient always creates a child allocator. While this is good in the general case to track memory leaks, this prevents usage scenarios where vectors live longer than the Flight client.

What about adding a FlightClient.Builder#createChildAllocator(boolean) that would allow users to choose whether they want a child allocator (current behavior which will be the default) or directly use the provided one?

I can create a PR for this.