Improve execution environment type validation

[Mrtenz]

This improves the type validation in execution-environments. The auto-generated OpenRPC types aren't great, for example:

export function isSnapRpcResult(obj: any, _argumentName?: string): obj is SnapRpcResult {
    return (
        true
    )
}

I replaced those with type guards from the @metamask/utils package, and some new type guards.

To do

• Add validators for getHandlerArguments.

[FrederikBolding]

Why do we need a union if it is optional?

[Mrtenz]

I think I ran into some issues otherwise, and needed this to keep it compatible.