Backend: Implement endpoint for the image annotations

[MercerFrey]

Description:

Image annotations should have Get operation. Since we will use Web Annotation Data Model, this endpoint should return image annotation in the following format.

{
            "@context": "http://www.w3.org/ns/anno.jsonld",
            "id": "http://example.org/anno20",
            "type": "Annotation",
            "body": {
                "type" : "TextualBody",
                "value" : "<p>example!</p>",
                "format" : "text/html",
                "language" : "en"
            },
            "target": {
                "source": "http://example.org/post-slug",
                "selector": {
                    "type": "FragmentSelector",
                    "conformsTo": "http://www.w3.org/TR/media-frags/",
                    "value": "xywh=50,50,640,480"
                }
            }
}

Deadline: 24.12.2022 23.59

[MercerFrey]

539 This pull request creates a GET request endpoint for the image annotation with given annotation id.