Feat implement WorkItemAttachment and add relations to WorkItem

[micha91]

This PR introduces the following features:

• Implement WorkItemAttachment datamodel and post/patch/get methods
• Make use of the relations field in the WorkItem to add stub links and attachments to the WorkItem
  • You have to add attachments or linkedWorkItems to the fields parameter to get them
  • Some fields are not included here and it seems like they cannot be added
• There is a possibility to calculate a checksum for WorkItems
  • This checksum also includes all attachments and linked workitems
  • The client has a flag, which you can set to add the checksum when posting or patching a WorkItem
  • On patch the checksum has already to be calculated in advance to avoid a wrong checksum calculation, if a partial patch is performed

[micha91]

@ewuerger TBD: Do we want to include the ID in the checksum calculation? There is one big downside of this: When we initially post a WorkItem, we don't have the ID, so we would have to patch a WorkItem always at least once after creation.

[ewuerger]

Do we want to include the ID in the checksum calculation? There is one big downside of this: When we initially post a WorkItem, we don't have the ID, so we would have to patch a WorkItem always at least once after creation.

I think it makes only sense to include data which can change in the checksum. IDs are stable, on the other side the uuid_capella is included and does not bring the additional update on creation. From me it's a no.

[ewuerger]

One tiny thing. Then: LGTM.