SOAP Source plugin

This would essentially depend on XML. Which has some working pieces already in migrate_plus and in migrate_source_xml.

I'm on a project now that requires importing from a SOAP service, so I'll be looking at this over the next couple of weeks. At first glance, this would probably be independent from our existing XML support, building on the PHP SoapClient class instead.

Very basic POC, known to work for exactly one service...

Yep, so responses aren't necessarily XML (my real-world scenario turns out to return an object containing an array of objects), so going to need a more flexible way to handle responses. Perhaps a SoapResponseParser plugin type - it's plugins all the way down!

This seems to work (so far) for my use case. For now, the different response types are hardcoded - the type-specific code is a line or two each, so introducing a plugin type seems overkill.

I've completed implementing the migration from SOAP for my current project - setting to "Needs review", if anyone else is doing SOAP migration please give it a whirl.

I've gone ahead and committed this - if anyone has any issues with it, please open a fresh issue.