Allow using custom IDirectoryFactory in Examine

[ljfio]

We have a requirement to store unique Lucene indexes on the same machine (through an Azure slot-swap scenario between our staging and production data connections, where the indexed content is different in each database).

Our workaround at the moment replaces the existing IApplicationIdentifier in the IServiceCollection to create a unique identifier for the temporary folder name whenever we change settings as part of the slot-swap.

Ideally it would be great to use our own custom IDirectoryFactory and configure Umbraco (10) to use it in the options here