[Feature Request] C++: Create arena based allocator and have access to it

### Describe the feature request

I would like to reuse a global allocator.

With api.CreateAndRegisterAllocatorV2 I can create global allocator, but I still have no access to it.

With api.CreateAllocator I can create one and have access to it, but it requires a session as a parameter.
So, I need to create a session with some dummy graph first.

### Describe scenario use case

With an access to allocator, I can reuse it to pre-allocate tensors (not with bindings), to know current utilization of a memory, etc.

It can be a new api.GetAllocator method or api.CreateAllocator without session

Start by tracing the api.CreateAndRegisterAllocatorV2 and api.CreateAllocator entry points and how allocator access currently depends on a session. Define the API behavior needed for a reusable global allocator, including pre-allocating tensors and inspecting utilization. Done means the chosen API provides access without requiring a dummy graph or session and is covered by relevant tests.

Written by the indexing model from the issue text.