ApiDerive.staking backwards compatibility.

[TarikGul]

This PR works on giving the support needed for backwards compatibility. With the recent staking breaking changes these PR's introduced: #5860 #5862 It also ended up breaking compatibility for some parachains in the UI.

This is part 1 of the staking changes saga, and once this goes in I can rework the UI to have support for the new and the old.

Summary

• ownExposure && ownExposures will properly build safe types for historic calls such as erasStakers, and erasStakersClipped. It also ensures the calls exist or else will default to the empty type.

• query && queryMulti now take in a flag withExposureErasStakersLegacy and returns a new field called exposureEraStakers

• nextElected now has support for both erasStakersPaged and erasStakers.

TODO:

• Test the following changes against apps to ensure it works for both old and new.

NOTE: My one concern is that api.derive.staking.stakersRewards* uses withClaimedErasRewards as a default flag here which could potentially still act as a regression for the UI.

rel: polkadot-js/apps#10512
rel :polkadot-js/apps#10513
rel: polkadot-js/apps#10505

[TarikGul]

It might be useful to add some throttling to api.derive.staking.queryMulti so that thousands of calls aren't emitted at the same time. (Not sure if this is the appropriate decision yet as I have not thought about it much.)

[TarikGul]

Note: I wont be able to test this against Apps until I have some stable internet come Sunday as I am still OOO.

[TarikGul]

I tested each call that was changed against polkadex to ensure the old calls are now available, and I also ensured everything is fault tolerant so it wont fail if the calls don't exist.

[polkadot-js-bot]

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