Add activity feeds API (#23494)

Close #5666

Add APIs for getting activity feeds.

modules/structs/activity.go

@@ -0,0 +1,22 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package structs
+
+import "time"
+
+type Activity struct {
+	ID        int64       `json:"id"`
+	UserID    int64       `json:"user_id"` // Receiver user
+	OpType    string      `json:"op_type"`
+	ActUserID int64       `json:"act_user_id"`
+	ActUser   *User       `json:"act_user"`
+	RepoID    int64       `json:"repo_id"`
+	Repo      *Repository `json:"repo"`
+	CommentID int64       `json:"comment_id"`
+	Comment   *Comment    `json:"comment"`
+	RefName   string      `json:"ref_name"`
+	IsPrivate bool        `json:"is_private"`
+	Content   string      `json:"content"`
+	Created   time.Time   `json:"created"`
+}