Define generic AbiExample for OnceLock (solana-labs#33520)

* Define generic AbiExample for OnceLock

* Guard with cfg...

cargo-test-sbf (governance/addin-mock/program, governance/program):
error[E0658]: use of unstable library feature 'once_cell'
   --> src/abi_example.rs:559:36
     |
 559 | impl<T: AbiExample> AbiExample for std::sync::OnceLock<T> {
     |                                    ^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #74465 <rust-lang/rust#74465> for more information
     = help: add `#![feature(once_cell)]` to the crate attributes to enable

frozen-abi/src/abi_example.rs

@@ -555,3 +555,10 @@ impl<O: AbiEnumVisitor, E: AbiEnumVisitor> AbiEnumVisitor for Result<O, E> {
         digester.create_child()
     }
 }
+
+#[cfg(not(target_os = "solana"))]
+impl<T: AbiExample> AbiExample for std::sync::OnceLock<T> {
+    fn example() -> Self {
+        Self::from(T::example())
+    }
+}

vote/src/vote_account.rs

@@ -1,5 +1,3 @@
-#[cfg(RUSTC_WITH_SPECIALIZATION)]
-use solana_frozen_abi::abi_example::AbiExample;
 use {
     itertools::Itertools,
     serde::ser::{Serialize, Serializer},
@@ -30,15 +28,15 @@ pub enum Error {
     InvalidOwner(/*owner:*/ Pubkey),
 }
 
-#[derive(Debug)]
+#[derive(Debug, AbiExample)]
 struct VoteAccountInner {
     account: AccountSharedData,
     vote_state: OnceLock<Result<VoteState, Error>>,
 }
 
 pub type VoteAccountsHashMap = HashMap<Pubkey, (/*stake:*/ u64, VoteAccount)>;
 
-#[derive(Clone, Debug, Deserialize)]
+#[derive(Clone, Debug, Deserialize, AbiExample)]
 #[serde(from = "Arc<VoteAccountsHashMap>")]
 pub struct VoteAccounts {
     vote_accounts: Arc<VoteAccountsHashMap>,
@@ -321,26 +319,6 @@ impl Serialize for VoteAccounts {
     }
 }
 
-#[cfg(RUSTC_WITH_SPECIALIZATION)]
-impl AbiExample for VoteAccountInner {
-    fn example() -> Self {
-        Self {
-            account: AccountSharedData::example(),
-            vote_state: OnceLock::from(Result::<VoteState, Error>::example()),
-        }
-    }
-}
-
-#[cfg(RUSTC_WITH_SPECIALIZATION)]
-impl AbiExample for VoteAccounts {
-    fn example() -> Self {
-        Self {
-            vote_accounts: Arc::<VoteAccountsHashMap>::example(),
-            staked_nodes: OnceLock::from(Arc::<HashMap<Pubkey, u64>>::example()),
-        }
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use {