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/issue.go

@@ -56,3 +56,20 @@ func (gitea *Gitea) GetIssues(repo Repository) ([]Issue, error) {
 	}
 	return data, nil
 }
+
+func (gitea *Gitea) GetIssue(repo Repository, id uint) (Issue, error) {
+	var issue Issue
+	url := fmt.Sprintf("%s/repos/%s/issues/%d", gitea.Url(), repo.Full_name, id)
+	response, err := http.Get(url)
+	if err != nil {
+		return issue, 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(&issue); err != nil {
+		return issue, err
+	}
+	return issue, nil
+}