Add API for Variables (#29520)

close #27801

---------

Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit 62b073e6f31645e446c7e8d6b5a506f61b47924e)

Conflicts:
	- modules/util/util.go
          Trivial resolution, only picking the newly introduced function
	- routers/api/v1/swagger/options.go
          Trivial resolution. We don't have UserBadges, don't pick that part.
	- templates/swagger/v1_json.tmpl
          Regenerated.

modules/structs/variable.go

@@ -0,0 +1,37 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package structs
+
+// CreateVariableOption the option when creating variable
+// swagger:model
+type CreateVariableOption struct {
+	// Value of the variable to create
+	//
+	// required: true
+	Value string `json:"value" binding:"Required"`
+}
+
+// UpdateVariableOption the option when updating variable
+// swagger:model
+type UpdateVariableOption struct {
+	// New name for the variable. If the field is empty, the variable name won't be updated.
+	Name string `json:"name"`
+	// Value of the variable to update
+	//
+	// required: true
+	Value string `json:"value" binding:"Required"`
+}
+
+// ActionVariable return value of the query API
+// swagger:model
+type ActionVariable struct {
+	// the owner to which the variable belongs
+	OwnerID int64 `json:"owner_id"`
+	// the repository to which the variable belongs
+	RepoID int64 `json:"repo_id"`
+	// the name of the variable
+	Name string `json:"name"`
+	// the value of the variable
+	Data string `json:"data"`
+}