chore(sdk): Add MaybeArbitrary as super trait

[zitup]

Closes #12582.

[emhane]

nice!

[emhane]

Suggested change

	});
	})

pls revert unrelated formatting change (use cargo +nightly fmt instead of cargo fmt)

[emhane]

instead of this bound here, relax the arbitrary impl for SealedHeader

#[cfg(any(test, feature = "arbitrary"))]
impl<'a, H> arbitrary::Arbitrary<'a> for SealedHeader<H> where H: for<'a> arbitrary::Arbitrary<'a> {
    fn arbitrary(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
        let header = Header::arbitrary(u)?;

        Ok(Self::seal(header))
    }
}