[FEATURE] Add support for indexing pressure stats

[dbwiddis]

Is your feature request related to a problem?

Coming from https://github.com/opensearch-project/opensearch-java/issues/453

The response for the nodes stats API only includes a subset of the available Nodes stats, notably missing the results for indexing_pressure (currently used by AD plugin) and shard_indexing_pressure.

What solution would you like?

Add parsing for the full set of Nodes stats.

What alternatives have you considered?

Manually parsing JSON.

Do you have any additional context?

See this blog post.

Happy to (try to) do this work in my spare time, may need some pointers and guidance on where to make the changes.