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>

tests/integration/api_settings_test.go

@@ -35,6 +35,7 @@ func TestAPIExposedSettings(t *testing.T) {
 		DefaultPagingNum:       setting.API.DefaultPagingNum,
 		DefaultGitTreesPerPage: setting.API.DefaultGitTreesPerPage,
 		DefaultMaxBlobSize:     setting.API.DefaultMaxBlobSize,
+		DefaultMaxResponseSize: setting.API.DefaultMaxResponseSize,
 	}, apiSettings)
 
 	repo := new(api.GeneralRepoSettings)