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>

routers/api/v1/swagger/options.go

@@ -118,6 +118,9 @@ type swaggerParameterBodies struct {
 	// in:body
 	EditAttachmentOptions api.EditAttachmentOptions
 
+	// in:body
+	GetFilesOptions api.GetFilesOptions
+
 	// in:body
 	ChangeFilesOptions api.ChangeFilesOptions