Add support for ArcGIS Online Map Services

[seansweeney]

My notes say we discussed this briefly at the 2017 GPV User Group meeting but I forgot to bring it up this year. I don't see that it's already been added as an issue.

I know this is a big change from the not-quite-exactly-deprecated SOAP API to the REST API, but we are making every effort to offload on-premises ArcGIS Server services to ArcGIS Online to reduce our hardware and software maintenance efforts, and right now GPV is around 1/4 of our Map Services.

I realize that this would mean the map and the stored procedures would be using separate data sources, and burden would be on us to make sure that map and data ID values are consistent, but I think we could make that work.

To me this is more important than OGC support because as I said we're looking to get away from self-hosting.