Fix MR Selection, Go Code Refactor (#358)

refactor: Refactors the Go codebase into a more modular and idiomatic approach
fix: require selection of specific MR when there are multiple targets for a given source branch
feat: Allows for the passing of Gitlab's filter options when choosing an MR, improves MR selection
feat: API to choose an MR from a list based on the provided username's involvement as an assignee/reviewer/author

This is a #MINOR release

cmd/app/assignee.go

@@ -0,0 +1,81 @@
+package app
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type AssigneeUpdateRequest struct {
+	Ids []int `json:"ids"`
+}
+
+type AssigneeUpdateResponse struct {
+	SuccessResponse
+	Assignees []*gitlab.BasicUser `json:"assignees"`
+}
+
+type AssigneesRequestResponse struct {
+	SuccessResponse
+	Assignees []int `json:"assignees"`
+}
+
+type assigneesService struct {
+	data
+	client MergeRequestUpdater
+}
+
+/* assigneesHandler adds or removes assignees from a merge request. */
+func (a assigneesService) handler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	if r.Method != http.MethodPut {
+		w.Header().Set("Access-Control-Allow-Methods", http.MethodPut)
+		handleError(w, InvalidRequestError{}, "Expected PUT", http.StatusMethodNotAllowed)
+		return
+	}
+
+	body, err := io.ReadAll(r.Body)
+	if err != nil {
+		handleError(w, err, "Could not read request body", http.StatusBadRequest)
+		return
+	}
+
+	defer r.Body.Close()
+	var assigneeUpdateRequest AssigneeUpdateRequest
+	err = json.Unmarshal(body, &assigneeUpdateRequest)
+
+	if err != nil {
+		handleError(w, err, "Could not read JSON from request", http.StatusBadRequest)
+		return
+	}
+
+	mr, res, err := a.client.UpdateMergeRequest(a.projectInfo.ProjectId, a.projectInfo.MergeId, &gitlab.UpdateMergeRequestOptions{
+		AssigneeIDs: &assigneeUpdateRequest.Ids,
+	})
+
+	if err != nil {
+		handleError(w, err, "Could not modify merge request assignees", http.StatusInternalServerError)
+		return
+	}
+
+	if res.StatusCode >= 300 {
+		handleError(w, GenericError{endpoint: "/mr/assignee"}, "Could not modify merge request assignees", res.StatusCode)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	response := AssigneeUpdateResponse{
+		SuccessResponse: SuccessResponse{
+			Message: "Assignees updated",
+			Status:  http.StatusOK,
+		},
+		Assignees: mr.Assignees,
+	}
+
+	err = json.NewEncoder(w).Encode(response)
+	if err != nil {
+		handleError(w, err, "Could not encode response", http.StatusInternalServerError)
+	}
+}