[Primitives] Make `NewBlock::signature` Typed

[frisitano]

I would like to have a solution that ensures correct typing, such that we have:

/// A message that is used to announce a new block to the network.
#[derive(Clone, Debug, PartialEq, Eq, RlpEncodable, RlpDecodable)]
pub struct NewBlock {
    /// The signature from the block author.
    pub signature: Signature,
    /// The block that is being announced.
    pub block: reth_scroll_primitives::ScrollBlock,
}

The reason we currently don't have this is because Signature does not implement Encodable. We may be able to address this by implementing Encodable ourselves on this type instead of leveraging the macro but I'm not sure. I will open an issue.

Originally posted by @frisitano in #68 (comment)

[greged93]

Link to the alloy issue: alloy-rs/core#938