add(gitea-api): individual issue and milestone getter

This implements the api getters for extracting a single issue and single
milestone for a given gitea repository and id. WIP for #1.

internal/gitea/milestone.go

@@ -36,3 +36,20 @@ func (gitea *Gitea) GetMilestones(repo Repository) ([]Milestone, error) {
 	}
 	return data, nil
 }
+
+func (gitea *Gitea) GetMilestone(repo Repository, id uint) (Milestone, error) {
+	url := fmt.Sprintf("%s/repos/%s/milestones/%d", gitea.Url(), repo.Full_name, id)
+	response, err := http.Get(url)
+	var milestone Milestone
+	if err != nil {
+		return milestone, err
+	}
+	defer response.Body.Close()
+	decoder := json.NewDecoder(response.Body)
+	// NOTE: remove this if I do not want to store everything from the json result in the struct
+	decoder.DisallowUnknownFields() // remain if every field shall be extracted
+	if err = decoder.Decode(&milestone); err != nil {
+		return milestone, err
+	}
+	return milestone, nil
+}