Implement subsetOf

[AlexJSully]

Implement the susbetOf method based on the N1 spec: https://hl7.org/fhirpath/N1/index.html#subsetofother-collection-boolean

5.1.8. subsetOf(other : collection) : Boolean
Returns true if all items in the input collection are members of the collection passed as the other argument. Membership is determined using the [= (Equals)](https://hl7.org/fhirpath/N1/index.html#equals) (=) operation.

Conceptually, this function is evaluated by testing each element in the input collection for membership in the other collection, with a default of true. This means that if the input collection is empty ({ }), the result is true, otherwise if the other collection is empty ({ }), the result is false.

The following example returns true if the tags defined in any contained resource are a subset of the tags defined in the MedicationRequest resource:

MedicationRequest.contained.meta.tag.subsetOf(MedicationRequest.meta.tag)

GitOrigin-RevId: 520ee835d324807ada9458b1e75563cddacb5818