Reward tier model: add slots column

[vikinatora]

Description

As per the new requirements we should remove minimumBid column and create a new slots column.
slots will be a jsonb array column holding objects with index and minimumBid properties. This object can be extended with more properties in the future.

Example column config:

[{
  index: 1,
  minimumBid: 0.5
}, {
  index: 2,
  minimumBid: 0.3
}, {
  index: 2,
  minimumBid: 0.2
}]

Purpose

The main purpose of these changes will be when a user is placing a bid in an active auction. In the bid popup we need to calculate which exact slot the bidder will potentially win and validate the user's bid amount. The bid amount should be greater or equal to the respective minimum bid of the slot.

Other changes

We will need to rework the edit and add reward tiers endpoints both on the FE and BE.

@alexandrumatei36 @taskudis @boris-bonin Let me know if I'm missing something