Idea - PerModuleHostingEnvironment

[dazinator]

To be done after #14

This is similar to what I do for PerTenantHostingEnvironment but it would be an additional sandbox around each module, that could isolate it's filesystem from other modules filesystems. I.e each module would be provided an IHostingEnvironment with its own IFileProviders that are a composite with the current tenants IHostingEnvironment IFileProvider This means each module has its own isolated file system, but also has a view of the tenants files (and the tenants files has a view of the hosts files but not other tenants). With this in place it would mean ModuleA would not be able to see ModuleB's files, but both modules can see files available at the tenant level. In terms of how this looks on disc, it just means each module has a particular folder that is kept private to itself, and each module also has visibility of a folder that is private to the current tenant. If a module wanted to create a file that was visible to other modules, it would need to create the file within the tenants directory and not its private folder.