Refactor response types

[Syndesi]

Currently responses contain the property type, which specifies how the response should be parsed.

Possible types include:

• _PartialCollection
• _TokenResponse
• _SearchResultResponse
• none, e.g. for GET /instance-configuration and GET /graph-structure
• dynamic, e.g. the element type itself for GET /<id>

Acceptance criteria:

• Should we return a type for non-element-responses at all?
• Should we normalize existing response types? If yes, we should store them in an enum.
• Documentation is updated?
• Tests are updated?