Remove `pallet-getter` usage from pallet-session (#4972)

As per #3326, removes usage of the `pallet::getter` macro from the
`session` pallet. The syntax `StorageItem::<T, I>::get()` should be used
instead.

Also, adds public functions for compatibility.

NOTE: The `./historical` directory has not been modified.

cc @muraca

polkadot address: 5GsLutpKjbzsbTphebs9Uy4YK6gTN47MAaz6njPktidjR5cp

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Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>