fix: Add serde_core::{de, ser}::{Deserialize, Serialize} to allowed-external-types.toml following the recent breaking changes introduced in serde

Author: lucamuscat

opentelemetry-otlp/allowed-external-types.toml

@@ -8,6 +8,9 @@ allowed_external_types = [
     # serde
     "serde::de::Deserialize",
     "serde::ser::Serialize",
+    ## serde: Required for serde version 1.0.222 and greater.
+    "serde_core::de::Deserialize",
+    "serde_core::ser::Serialize",
     # tonic is a pre 1.0 crate
     "tonic::metadata::map::MetadataMap",
     "tonic::transport::channel::tls::ClientTlsConfig",