Allow YAML aliases when using `yaml_safe_classes`

We need to allow aliases in our YAML parsing.

The standard YAML emitter will introduce aliases in its output if there are repeated references in nested structures. Some of these are unavoidable, e.g. multiple references to the same date object. Sample YAML (observed in practice) might contain something like:

attendances:
- appointmentdate: &1 !ruby/object:DateTime 2023-01-01 00:00:00.000000000 Z
  appointmenttime: '11:15:00'
- appointmentdate: *1
  appointmenttime: '11:15:00'

NHSDigital / ndr_support

Allow YAML aliases when using `yaml_safe_classes` #46