fix: modify Platform trait

[alessandromazza98]

This PR makes it possible to actually return the Platform specific context in the build_payload fn of the Platform trait.

This way you can use the platform specific context there to do what you need. Otherwise you get a generic P::CheckpointContext type that you cannot really use.

Before this PR:

fn build_payload<P>(
    payload: Checkpoint<P>,
    provider: &dyn StateProvider,
    ) -> Result<types::BuiltPayload<P>, PayloadBuilderError>
    where
    P: PlatformExecBounds<Self>,
    {
        // here is just the generic P::CheckpointContext
        let my_context = payload.context();
        ...
}

With this PR:

fn build_payload<P>(
    payload: Checkpoint<P>,
    provider: &dyn StateProvider,
    ) -> Result<types::BuiltPayload<P>, PayloadBuilderError>
    where
    P: PlatformExecCtxBounds<Self>,
    {
        // here is the very specific type, and you access all its inner elements
        let my_context = payload.context();
        ...
}

[alessandromazza98]

can you merge it? or is there any question? thanks