Add API for gitignore templates (#22783)

This implements the [Gitignores template API of GitHub](https://docs.github.com/en/rest/gitignore?apiVersion=2022-11-28) in Gitea

tests/integration/api_gitignore_templates_test.go

@@ -0,0 +1,53 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"fmt"
+	"net/http"
+	"testing"
+
+	"code.gitea.io/gitea/modules/options"
+	repo_module "code.gitea.io/gitea/modules/repository"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAPIListGitignoresTemplates(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	req := NewRequest(t, "GET", "/api/v1/gitignore/templates")
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	// This tests if the API returns a list of strings
+	var gitignoreList []string
+	DecodeJSON(t, resp, &gitignoreList)
+}
+
+func TestAPIGetGitignoreTemplateInfo(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	// If Gitea has for some reason no Gitignore templates, we need to skip this test
+	if len(repo_module.Gitignores) == 0 {
+		return
+	}
+
+	// Use the first template for the test
+	templateName := repo_module.Gitignores[0]
+
+	urlStr := fmt.Sprintf("/api/v1/gitignore/templates/%s", templateName)
+	req := NewRequest(t, "GET", urlStr)
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	var templateInfo api.GitignoreTemplateInfo
+	DecodeJSON(t, resp, &templateInfo)
+
+	// We get the text of the template here
+	text, _ := options.Gitignore(templateName)
+
+	assert.Equal(t, templateInfo.Name, templateName)
+	assert.Equal(t, templateInfo.Source, string(text))
+}