Remove `additional` from `TokenValidationRule`

# Feature Request

The `TokenValidationRule` interface method gets an `additional` field that never gets set by anyone, or, better, it gets set with the value of `TokenRepresentation.getAdditional()`, but those additional properties are only set in egress when a new JWT token is issued, plus the other point sof access are in the data-plane, so in the future also the `additional` field in the `TokenRepresentation` could be removed as well.

## Which Areas Would Be Affected?

_e.g., DPF, CI, build, transfer, etc._

## Why Is the Feature Desired?

_Are there any requirements?_

## Who will sponsor this feature?

_Please @-mention the committer that will sponsor your feature_.

## Solution Proposal

_If possible, provide a (brief!) solution proposal._