Implement subsidy adjustment for USDt

[ebma]

Context

Currently, the rate adjustment calculation that happens on Pendulum is assuming that the axlUSDC price is similar to the price of USDT. However, if the price of these two tokens deviate, the effective rate will be off. The goal of this ticket is to increase the accuracy of this adjustment.

TODO

Take the axlUSDC <> USDT price into account when doing the rate adjustment calculation. The goal is to have the effective rate close to the configured adjusted rate.