Transforms a custom field value by letting the custom field tell us what the value (and optionally parent value)
should be transformed into. The resulting ExternalCustomFieldValue will have a null id.
projectImportMapper - a fully populated and validated project import mapper
externalCustomFieldValue - the value representing a single custom field value from the backup XML
newProjectId - the id of the project, in this JIRA instance, that the data is being imported into
an ExternalCustomFieldValue that contains the transformed values and a null id, null if the custom
field has been flagged as ignored by the CustomFieldMapper, or if the custom field thinks the value should
be transformed to null, or if for some reason the issue id has not been mapped.