Implement validator list.

[zie1ony]

• Admin controls the list of validators. Can add and remove validators.
• On staking, random validator is selected from the list and the delegation is made to him.
• When admin removes the validator from the list, then tokens are delegated equally to all validators on the list.
  • introduce a new method: redistribute to handle undelegated tokens after 7 eras.
• Handle case where validator stop being validator and tokens get back to staking purse.
• Edgecase: there might be zero validators on the list. When last validator is removed, then sCSPR holders should still be able to withdraw their CSPR.