sp-core: impl serde for some offchain types (#11512)

* sp-core: impl serde for some offchain types

* Update primitives/core/src/offchain/mod.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* remove serde impls from OpaqueNetworkState/OpaqueMultiaddr

* derive default

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/core/src/offchain/mod.rs

@@ -208,12 +208,14 @@ impl OpaqueMultiaddr {
 #[derive(
 	Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Default, RuntimeDebug, PassByInner, Encode, Decode,
 )]
+#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
 pub struct Timestamp(u64);
 
 /// Duration type
 #[derive(
 	Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Default, RuntimeDebug, PassByInner, Encode, Decode,
 )]
+#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
 pub struct Duration(u64);
 
 impl Duration {