Enable search on several resource types

[Jasopaum]

Hello!
We started to use fhirpath but had several features to deliver that were not possible in the current version. That's why we started to work on a fork. Unfortunately, things got a little messy but we're in the process of trying to clean what we did and chunk it into "atomic" PRs. Here's the first one 🙂

Here, we try to make it possible to search on several resource types instead of just one. I don't know if you had an idea of how you wanted to do that. We chose to add this functionality by changing as less code as we could but maybe it wasn't the best way.

There are some issues that we may need to fix before merging:

• We broke the field selection with fhirpath (I think it's mandatory to fix this one).
• We had to change the API of some classes/functions of yours.

Anyway, we'd love some feedback on what we did even if it implies more work on our side!

[nazrulworld]

@Jasopaum Thank you so much for the great effort. Looks everything fine! just some minor issue

1. all tests should be passed (but if you want, we can merge it now and make broken tests working together) because that will kind of make sure other existing providers will not break.
2. please add your name in AUTHOR.rst and update CHANGES.rst

[nazrulworld]

Thanks a lot.