[pallet-revive] Fix `caller_is_root` return value (#7086)

Closes https://github.com/paritytech/polkadot-sdk/issues/6767.

The return type of the host function `caller_is_root` was denoted as
`u32` in `pallet_revive_uapi`. This PR fixes the return type to `bool`.

As a drive-by, the PR re-exports `pallet_revive::exec::Origin` to extend
what can be tested externally.

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Co-authored-by: Cyrill Leutwiler <bigcyrill@hotmail.com>