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

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

modules/structs/settings.go

@@ -26,6 +26,7 @@ type GeneralAPISettings struct {
 	DefaultPagingNum       int   `json:"default_paging_num"`
 	DefaultGitTreesPerPage int   `json:"default_git_trees_per_page"`
 	DefaultMaxBlobSize     int64 `json:"default_max_blob_size"`
+	DefaultMaxResponseSize int64 `json:"default_max_response_size"`
 }
 
 // GeneralAttachmentSettings contains global Attachment settings exposed by API