MimeTypeTerminologyService should handle STU3 canonical for MimeType ValueSet

[alexzautke]

In STU3 the canonical for the MimeType ValueSet, e.g. in Attachment.contentType is "http://www.rfc-editor.org/bcp/bcp13.txt" not "http://hl7.org/fhir/ValueSet/mimetypes". This should be handled by the the terminology service as an additional canonical. Note that this is a bit more tricky as there is not a ValueSet resource defined by the spec itself. It's just a binding on the datatype itself as far as I can see. This might cause some issues in the VS resolver.