feat: introduce yield rate tiers

[igorsenych-cw]

Main changes

This pull request updates the yield calculation mechanism by replacing the YieldRate with two new structs: YieldTieredRate and RateTier. This change introduces tiered yield rates with balance caps, allowing different portions of a user's balance to accrue yield at different rates.

New structures declaration

    /**
     * @dev Structure representing a yield rate schedule that becomes effective from a specific day.
     * Contains multiple tiers, each applying a specific rate up to a balance cap.
     *
     * Fields:
     * - `effectiveDay`: The day index from which this yield rate schedule becomes effective.
     * - `tiers`: An array of `RateTier` structs defining the rate tiers.
     */
    struct YieldTieredRate {
        RateTier[] tiers;
        uint16 effectiveDay;
    }

    /**
     * @dev Structure representing a yield rate tier within a schedule.
     * Applies a specific rate to a portion of the balance up to a specified cap.
     *
     * Fields:
     * - `rate`: The yield rate value (scaled by RATE_FACTOR).
     * - `cap`: The maximum balance amount for which this rate applies.
     */
    struct RateTier {
        uint48 rate;
        uint64 cap;
    }

Yield calculation adjustments:

Yield is now calculated by applying different rates to different segments of a user's balance based on the defined tiers.
The calculation functions have been updated to iterate over the RateTier array within each YieldTieredRate.

Example:

• Tier 1: Up to 500 BRLC, the yield is calculated at a 5% rate.
• Tier 2: For the next 1000 BRLC, the yield is calculated at a 3% rate.
• Tier 3: A 1% yield rate is applied for the rest of the balance.

Test Coverage

The new changes have yet to be tested.