Auto merge of #15519 - gterzian:implement_structuredclone_callbacks, r=jdm

implement structured clone callbacks - support Blob cloning

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1. Implement stubs for structured clone callbacks.
2. Support Blob cloning.

Partial implementation of https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15021 (github issue number if applicable).

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- [ ] These changes do not require tests because _____

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components/script/dom/blob.rs

@@ -163,6 +163,11 @@ impl Blob {
         }
     }
 
+    /// Get a copy of the type_string
+    pub fn get_type_string(&self) -> String {
+        self.type_string.clone()
+    }
+
     /// Get a FileID representing the Blob content,
     /// used by URL.createObjectURL
     pub fn get_blob_url_id(&self) -> Uuid {