Add API endpoint to request contents of multiple files simultaniously (#34139)

Adds an API POST endpoint under `/repos/{owner}/{repo}/file-contents`
which receives a list of paths and returns a list of the contents of
these files.

This API endpoint will be helpful for applications like headless CMS
(reference: https://github.com/sveltia/sveltia-cms/issues/198) which
need to retrieve a large number of files by reducing the amount of
needed API calls.

Close #33495

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/api.go

@@ -18,6 +18,7 @@ var API = struct {
 	DefaultPagingNum       int
 	DefaultGitTreesPerPage int
 	DefaultMaxBlobSize     int64
+	DefaultMaxResponseSize int64
 }{
 	EnableSwagger:          true,
 	SwaggerURL:             "",
@@ -25,6 +26,7 @@ var API = struct {
 	DefaultPagingNum:       30,
 	DefaultGitTreesPerPage: 1000,
 	DefaultMaxBlobSize:     10485760,
+	DefaultMaxResponseSize: 104857600,
 }
 
 func loadAPIFrom(rootCfg ConfigProvider) {