FHIRPath ResolveFunction should work over a collections with multiple items

[lmsurpre]

Describe the bug
US Core 4.0 defines the following contraint expression at http://hl7.org/fhir/us/core/STU4/StructureDefinition-us-core-provenance.html#constraints

((%resource.agent.who.resolve() is Practitioner) or (%resource.agent.who.resolve() is Device)) implies exists()

Currently, when our FHIRPath engine evaluates that against input with multiple agents, our resolve() function implementation rejects that:

java.lang.IllegalArgumentException: Input collection has 2 items, but only 1 is allowed

However, the definition at https://build.fhir.org/fhirpath.html#functions says that this should work over a collection:

resolve() : collection

For each item in the collection, if it is a string that is a uri (or canonical or url), locate the target of the reference, and add it to the resulting collection. If the item does not resolve to a resource, the item is ignored and nothing is added to the output collection.

The items in the collection may also represent a Reference, in which case the Reference.reference is resolved.

Environment
main

To Reproduce
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Expected behavior
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Additional context
This particular constraint from US Core has other issues... https://hl7.org/fhirpath/#is-type-specifier says

If the input collections contains more than one item, the evaluator will throw an error.

So even if we fixed this issue in our resolve function, the constraint would still fail because we have a non-singleton collection on the lhs of the or operator.

I opened https://jira.hl7.org/browse/FHIR-36328 for this part of it, but we should consider patching this expression in our packaging of us core 4.0 when we fix this issue.

[lmsurpre]

Opened #3490 for patching the fhirpath expression

[lmsurpre]

I went to "fix" this and I found that our resolve function ALREADY supports multiple nodes in the input collection. The error from the description is actually coming from the is operator and this operator is correct to throw when there are multiple nodes as input.

I still opened a PR with the simple test I used to prove its working.