Make JSON-RPC error data property optional

[Mrtenz]

This fixes an issue where the data property of JsonRpcError wasn't properly inferred as optional, but rather as unknown. This is due to an issue(?) in TypeScript, where unknown | undefined is inferred simply as unknown, so Superstruct can't properly make the (type of the) field optional.

It's fixed by adding a new helper type, OptionalField, which explicitly makes a property (or multiple properties) of a type optional.

type JsonRpcError = Infer<typeof JsonRpcErrorStruct>; // { ..., data: unknown }
type JsonRcpError = OptionalField<Infer<typeof JsonRpcErrorStruct>, 'data'>; // { ..., data?: unknown }

This does not affect actual runtime validation of the types.