SIP-031 Coinbase management for last phase

[rdeioris]

The last phase of SIP-031 will release the remaining 300M STX in various chunks (defned in the SIP) at every tenure. To accomplish this we can associate the minting and transferring to the boot contract in the logic already managing the coinbase.

It is important to associate the additional mint event to the coinbase itself.

A map of the amount of STX to release based on the bitcoin height (already available in the SIP) will be hardcoded in rust.