Feature suggestion: add esrisearch() and other functions to support Portal Directory API

[elipousson]

I just added a new function to my esri2sf fork that works with the Search endpoint of the Portal Directory API (aka Sharing API) called esrisearch().

I haven't added any corresponding tests yet but I did add examples and, overall, it seems to work pretty well. If it isn't out of scope for this package, it may be useful to identify any other endpoints from this API that may be helpful to support in order to ensure consistency across the different possible functions.