[Lease] Variable premium interest rate

[gmanev7]

A new component of the total interest rate is going to be added, named a premium percent. It would vary across different leases depending on the staked NLS tokens.

The premium percent is determined at lease open time and is a function of the total staked Nls by the customer at that time. The more staked NLS the smaller the premium rate. The function would resemble the scale used at the Treasury contract for the calculation of the lenders' rewards in NLS. The scale should be modifiable with gov proposals.

The amounts would be sent to the Profit contract.