feat: params.ExtraPayloads.SetOnChainConfig() + Rules equiv

[ARR4N]

Why this should be merged

Idiomatic API for setting an extra payload:

extras := params.RegisterExtras(Extras[Foo,...]{})

var x ChainConfig
extras.SetOnChainConfig(&x, Foo{...})
// instead of
*extras.PointerFromChainConfig(&x) = Foo{...}

How this works

Just a convenience wrapper of the "instead of" line above.

The type ExtraPayloadGetter is now called ExtraPayloads because it's not just a getter. All references to getter are changed to extras or payloads, depending on what felt most appropriate.

How this was tested

Existing unit test changed to work via the wrapper.