## Why this should be merged
There are 3 places at which we perform the same, sensitive logic to
access registered payloads and as we modify more types this is likely to
expand. (e.g. `types.Header`).
## How this works
Introduces `pseudo.Accessor` to abstract the reusable code.
## How this was tested
Existing unit tests. Note that the `types.StateAccount` tests needed a
minor refactor to provide the assertions with access to the
`ExtraPayloads[T]` without introducing generic types anywhere.
## Why this should be merged
Provides access to extra payloads registered on `types.SlimAccount`, not
just on regular `types.StateAccount`. This is a breaking syntax change
to have the method reflect the behaviour.
## How this works
Modify existing method to accept the `ExtraPayloadCarrier` interface
instead of `*StateAccount`. The interface is implemented by both
`*StateAccount` and `*SlimAccount`.
## How this was tested
Covered by existing testing.
* feat: `state.{Get,Set}Extra[SA any](*StateDB,types.ExtraPayloads,...)`
* test: `GetExtra()` at each point in `CreateAccount()` + `SetExtra()` lifecycle
* test: reverting extras to snapshot
* test: `GetExtra()` after `StateDB.Copy()` and writes to original
