Add nominations to the `GET /accounts/{accountId}/staking-info` endpoint as per stated in the docs

[IkerAlus]

The docs state that the GET /accounts/{accountId}/staking-info endpoint returns and entry for the nominations of the queried stash account.

As we can see in the relevant code of the endpoint, this is not the case right now. We should make sure the entry is added into the endpoint output, to inform the user about the active nominations of the relevant stash account.