[Staking] Runtime api if era rewards are pending to be claimed (#4301)

closes https://github.com/paritytech/polkadot-sdk/issues/426.
related to https://github.com/paritytech/polkadot-sdk/pull/1189.

Would help offchain programs to query if there are unclaimed pages of
rewards for a given era.

The logic could look like below

```js
// loop as long as all era pages are claimed.
while (api.call.stakingApi.pendingRewards(era, validator_stash)) {
  api.tx.staking.payout_stakers(validator_stash, era)
}
```

prdoc/pr_4301.prdoc

@@ -0,0 +1,13 @@
+title: New runtime api to check if a validator has pending pages of rewards for an era.
+
+doc:
+  - audience:
+    - Node Dev
+    - Runtime User
+    description: |
+      Creates a new runtime api to check if reward for an era is pending for a validator. Era rewards are paged and this
+      api will return true as long as there is one or more pages of era reward which are not claimed.
+
+crates:
+- name: pallet-staking
+- name: pallet-staking-runtime-api