Method to check if file format upgrade is possible

[kneth]

Describe your problem or use case

In order to prevent the situation described in #7139, a method to check if a file format upgrade can be perform would be useful.

With the method, SDKs can throw an exception if the SoftResetFile schema mode is set - preventing unexpected data loss.

Describe the solution you'd like

bool SharedRealm::canUpgradeFileFormat(std::string path);

Additional context

[jedelbo]

The question is if deleting the file in this scenario is the right thing to do.

[jedelbo]

I have chosen just not to delete the file. Whether the file should be upgraded or not has nothing to do with the schema.