Add X-PublicFolderMailbox header for public folder access

[ecederstrand]

According to https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/public-folder-access-with-ews-in-exchange#routing-public-folder-requests, setting the X-PublicFolderMailbox HTTP header is necessary in some circumstances.

The logic in deciding when to set it and what to put as the value is described in the link, and the routing possibilities are detailed in https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-route-public-folder-hierarchy-requests and https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-route-public-folder-content-requests

Spoiler: it's non-trivial.