Fix get_header JSON deserialization. (#8228)

#8224


  Please list or describe the changes introduced by this PR.


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

beacon_node/builder_client/Cargo.toml

@@ -12,3 +12,7 @@ reqwest = { workspace = true }
 sensitive_url = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+
+[dev-dependencies]
+mockito = { workspace = true }
+tokio = { workspace = true }