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_files_change_test.go

@@ -44,13 +44,13 @@ func getChangeFilesOptions() *api.ChangeFilesOptions {
 		},
 		Files: []*api.ChangeFileOperation{
 			{
-				Operation: "create",
-				Content:   newContentEncoded,
+				Operation:     "create",
+				ContentBase64: newContentEncoded,
 			},
 			{
-				Operation: "update",
-				Content:   updateContentEncoded,
-				SHA:       "103ff9234cefeee5ec5361d22b49fbb04d385885",
+				Operation:     "update",
+				ContentBase64: updateContentEncoded,
+				SHA:           "103ff9234cefeee5ec5361d22b49fbb04d385885",
 			},
 			{
 				Operation: "delete",