Refactor "Content" for file uploading (#25851)

Before: the concept "Content string" is used everywhere. It has some
problems:

1. Sometimes it means "base64 encoded content", sometimes it means "raw
binary content"
2. It doesn't work with large files, eg: uploading a 1G LFS file would
make Gitea process OOM

This PR does the refactoring: use "ContentReader" / "ContentBase64"
instead of "Content"

This PR is not breaking because the key in API JSON is still "content":
`` ContentBase64 string `json:"content"` ``

tests/integration/api_repo_file_update_test.go

@@ -44,7 +44,7 @@ func getUpdateFileOptions() *api.UpdateFileOptions {
 			},
 			SHA: "103ff9234cefeee5ec5361d22b49fbb04d385885",
 		},
-		Content: contentEncoded,
+		ContentBase64: contentEncoded,
 	}
 }