feat: add RadioPayload trait alias and requirements

[hopeyen]

Description

Made a custom trait for more generic usage of the payload field for GraphcastMessage

pub trait RadioPayload:
    Message
    + ethers::types::transaction::eip712::Eip712<Error = Eip712Error>
    + Default
    + Clone
    + 'static
    + Serialize
    + async_graphql::OutputType 
{
  fn valid_outer(&self, outer: &GraphcastMessage<Self>) -> Result<&Self, MessageError>;
}

Renamed BuildMessageError to MessageError enum

Issue link (if applicable)

Part of https://github.com/graphops/engineering-meta/issues/39

Checklist

• Are tests up-to-date with the new changes?
• Are docs up-to-date with the new changes? (Open PR on docs repo if necessary)

[axiomatic-aardvark]

If this is something we expect all custom Radio message types to implement, how custom should/could the impl be? If we expect valid_outer to always be the same, shouldn't we enforce it on SDK side?

[hopeyen]

We should require all Radios to check if the inner payload is consistent with the general Graphcast message, although I don't expect them to always be the same. For example SimpleMessage which doesn't contain fields like nonce even though it is more of an example message type. Since we don't have a restriction on the fields for the payload type, I don't think it will be easy to check the fields between the inner and outer types.
Let me know if you think we should remove valid_outer altogether; maybe there's some other way to refactor the messages so the outer type doesn't have overlapping fields (currently identifier, nonce, graph_account, etc must be in inner for the signature and they are on the outer type for interacting with gossips and generic validations

[axiomatic-aardvark]

ah this makes total sense, in the current setup this is the best way then, thank you for explaining! 💯