add acceptUnknownHL7Extensions flag

[grahamegrieve]

from https://jira.hl7.org/browse/FHIR-31721:

Will submit a Git issue asking for a flag to be added to the validator that says "acceptUnknownHL7Extensions" that will handle the special case of the stricter validation that we apply to HL7-defined extensions that would choke at seeing extensions from an unrecognized version. This same extension should also make extension validation more lenient - i.e. don't presume that the definition available in the current version for the HL7 extension is the right one and treat validation errors against a defined extension or missing extension as warnings rather than errors.

I have no idea what this is supposed to do though. The way it's described doesn't match how the validator actually works, so nothing will happen unless someone/ some committee provides a clearer description that matches how the validator works. e.g why would there be an unrecognised version? Why wouldn't the definition be the right one? Why wouldn't there be errors if the extension is wrong?

[stale]

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