sp-core: Rename VrfOutput to VrfPreOutput (#2534)

This will make more sense after
https://github.com/paritytech/polkadot-sdk/pull/2524 since the
schnorrkel type for VRF outputs is also renamed in the latest version.
Can be reviewed independently though.

Can be merged after https://github.com/paritytech/polkadot-sdk/pull/1577
so that there is less pain for @davxy

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