remove pallet::getter from pallet-staking (#6184)

# Description

Part of https://github.com/paritytech/polkadot-sdk/issues/3326
Removes all pallet::getter occurrences from pallet-staking and replaces
them with explicit implementations.
Adds tests to verify that retrieval of affected entities works as
expected so via storage::getter.

## Review Notes

1. Traits added to the `derive` attribute are used in tests (either
directly or indirectly).
2. The getters had to be placed in a separate impl block since the other
one is annotated with `#[pallet::call]` and that requires
`#[pallet::call_index(0)]` annotation on each function in that block. So
I thought it's better to separate them.

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Co-authored-by: Dónal Murray <donal.murray@parity.io>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>