Add equivalent of CollectionsMarshal.GetValueRef for PooledDictionary.

[grokys]

Is your feature request related to a problem? Please describe.

.NET 6 exposes a low-level method to get a ref to the entry in a Dictionary<,>: dotnet/runtime#49388

It would be nice to have an equivalent to this in the pooled collections.

Describe the solution you'd like

Something like PooledCollectionsMarshal.GetValueRefOrNullRef?