Add support for tracking memory consumption of selected structures

[aschran]

Summarizing input from François:

Look into integrating a trait-based crate for tracking heap memory usage, perhaps:

1. https://github.com/paritytech/parity-common/tree/master/parity-util-mem
2. https://github.com/Aeledfyr/deepsize/

Validate the approach by carefully instrumenting a set of types on Narwhal with trait implementations (ideally derived as often as possible), up to what's needed to track the memory usage of the following table: https://github.com/MystenLabs/narwhal/blob/main/consensus/src/consensus.rs#L33

Export the memory usage gauge in a metric.

Related work in Sui: https://github.com/MystenLabs/sui/issues/3835