Add getGraphStatistics method and GraphStatistics class and it's exported C API

[kpango]

The following APIs have been newly added and internally modified in this PR because there are use cases where the contents of the ngt info index command are also used via CAPI.

New APIs

• CAPI:

  • NGTGraphStatistics struct

    • This is an export structure with members equivalent to C++'s GraphStatistics.

  • ngt_get_graph_statistics

    • CAPI that takes an NGTIndex and returns NGTGraphStatistics.

  • ngt_free_graph_statistics

    • CAPI that purges NGTGraphStatistics allocation.

• C++:

  • GraphStatistics class

    • An object with a collection of graph statistics, with Getter and Setter, and a toString method for Print.

  • getGraphStatistics

    • Takes an NGT::GraphIndex, constructs GraphStatistics based on the statistics, and returns it.

[masajiro]

Thanks!