API to check whether two files have the same schema

[wonder-sk]

When we need to check if two geopackages have the same structure, right now one needs to use GEODIFF_schema() twice and then compare JSON output - it would be nicer and more efficient if this was encapsulated in a single API call (and without the intermediate JSON output step)

E.g. bool GEODIFF_hasEqualSchema(file1, file2)

It would be useful if we could also report why the schema is not the same - user may have a hard time to understand what's wrong without extra hint like "Table ABC: new column XY"