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/git_blob.go

@@ -5,9 +5,9 @@ package structs
 
 // GitBlobResponse represents a git blob
 type GitBlobResponse struct {
-	Content  string `json:"content"`
-	Encoding string `json:"encoding"`
-	URL      string `json:"url"`
-	SHA      string `json:"sha"`
-	Size     int64  `json:"size"`
+	Content  *string `json:"content"`
+	Encoding *string `json:"encoding"`
+	URL      string  `json:"url"`
+	SHA      string  `json:"sha"`
+	Size     int64   `json:"size"`
 }