Provide Access to Reader's Current Memory Usage

[rmarrowstone]

I've been working on integrating Ion with Trino (fwiw trinodb/trino#24511 ).

The Trino Decoding APIs want to know the total memory currently used by the underlying decoder. So I would like a method to get an estimate of used memory by an IonReader. That would include any data buffers and symbol table/encoding context memory usage.

I expect that this could be some sort of Facet, like MemoryUsageAware idk.

[tgregg]

Consider whether this fills the need: https://github.com/openjdk/jol