add script to get corpus stats

[khieta]

This PR adds a small Python script to get stats about the corpus. The more complete solution would be to make a Rust crate that does proper parsing/analysis of policies/schemas, but I think this script will do for now.

Analysis of the current corpus in main

Total test cases	1982
Unique policies	1048 ( 52.9% )
Unique schemas	631 ( 31.8% )
Unique entity stores	918 ( 46.3% )
Entities per entity store	mean: 1 / median: 1 / p90: 2
Schema entity types	mean: 1 / median: 1 / p90: 1
Schema actions	mean: 1 / median: 1 / p90: 1
Trivial policies	834 ( 42.1% )
Pure RBAC (non-trivial) policies	436 ( 22.0% )
Pure ABAC (non-trivial) policies	60 ( 3.0% )

Comparison of the new vs. old corpus from #7

	Original	New	Difference
Total test cases	6065	6969	904
Unique policies	3486 ( 57.5% )	3918 ( 56.2% )	432
Unique schemas	2298 ( 37.9% )	2818 ( 40.4% )	520
Unique entity stores	2826 ( 46.6% )	3207 ( 46.0% )	381
Entities per entity store	mean: 1 / median: 1 / p90: 4	mean: 1 / median: 1 / p90: 7	n/a
Schema entity types	mean: 1 / median: 1 / p90: 2	mean: 1 / median: 1 / p90: 4	n/a
Schema actions	mean: 1 / median: 1 / p90: 2	mean: 1 / median: 1 / p90: 2	n/a
Trivial policies	2191 ( 36.1% )	2690 ( 38.6% )	499
Pure RBAC (non-trivial) policies	1148 ( 18.9% )	1299 ( 18.6% )	151
Pure ABAC (non-trivial) policies	156 ( 2.6% )	119 ( 1.7% )	-37

[khieta]

Ready for review! Scripts looks like they're working as expected -- you can find the output in the Action details. I'll update the main branch corpus and submodules in a future PR.