Support for FHIR version R4B

[kennethmyhra]

Describe the solution you'd like Support FHIR version R4B - http://hl7.org/fhir/2021Mar/ballot-intro.html

• [ ] Base it off the branch r4/master, name the new branch r4b/master
• [ ] Upgrade to firely-net-sdk 3.8 R4B (or later) pre-requisite: #379
• [ ] Support the generic search parameter _source

[andy-a-o]

Blocked by https://github.com/FirelyTeam/firely-net-sdk/issues/1746

[pietew]

Any reason why support for R4B has been blocked? :-)

[kennethmyhra]

@pietew It's not really blocked anymore except for our integration tests failing (consult PR #508) which I haven't had time to investigate.

One option is to verify the branch in #508 locally, and if it works satisfactory disable the integration tests for the time being for R4B.

If anyone wants to look into verifying R4B before I have the time to get back to it they are welcome to do that, and I will gladly review it.