[API Proposal]: ObjectPool.Create without new() constraint

### Background and motivation

Today the existing `ObjectPool.Create` has `new()` constraint on the generic type parameter, because `DefaultPooledObjectPolicy` requires it.

Suggestion is a non-nullable `IPooledObjectPolicy` overload, so with a custom pooled object policy, we can create object pools for `T` does not have a default ctor (that would be the benefit of creating a custom `IPooledObjectPolicy`)

```csharp
public static ObjectPool Create(IPooledObjectPolicy policy) where T : class
```

### API Proposal

```csharp
namespace Microsoft.Extensions.ObjectPool;

public static class ObjectPool
{
public static ObjectPool Create(IPooledObjectPolicy policy) where T : class
}
```

### API Usage

```csharp
ObjectPool pool = ObjectPool.Create(new MyTypePolicy());

class MyType(int value) { }

class MyTypePolicy : IPooledObjectPolicy
{
public MyType Create() => new MyType(0);
public bool Return(MyType obj)
{
return true;
}
}
```

### Alternative Designs

Instead of an overload (which might require changing and removing the parameter of the existing method), have a separate method name.

(I am happy to address the issue if there are no objections during the discussion.)