fix!: Extract-reqtypes-from-generic-serializer

[ewuerger]

The requirements text is outsourced from the generic serializer into it own serializer called:
add_requirements_text_grouped_by_type.

Still needs:

• Config validation
• Test case for the attachments from ReqIFText
• Documentation

The config should look like this:

Property:
  serializer:
    add_requirements_text_grouped_by_type:
      - Assumption
      - Rationale
      - Standards

Which adds custom fields Assumption, Rationale and Standards to the work item Property. The value of these custom fields is the unordered list of all texts from requirements of this type in relation to the Property.