Create a tool that prints an HNSW graph degree histogram

### Description

In luceneutil KnnGraphTester we have some code that will walk an HNSW graph and gather connectivity statistics (how many nodes are connected to each node). It would be nice to integrate this with a Lucene tool like CheckIndex so that we have a way to explore existing indexes that are not tied to the graph testing tools in luceneutil

Start by reading the HNSW graph connectivity-statistics code in luceneutil KnnGraphTester and the existing CheckIndex tool. Trace how CheckIndex examines existing indexes, then determine where the graph degree histogram belongs. Done means a Lucene tool can inspect an existing index and print HNSW graph degree statistics.

Written by the indexing model from the issue text.