Migrations

[brozorec]

When a migration is present, it needs to be executed after the upgrade, i.e. call the migrate function with a specific migration data.

1. Detect the version is migratable, i.e. check whether migrate() is there.
2. Propose to deploy a “proxy” upgrader only once → https://github.com/OpenZeppelin/stellar-contracts/blob/main/examples/upgradeable/upgrader/src/contract.rs
3. Store it locally similar to the npm package for Solidity → https://docs.openzeppelin.com/upgrades-plugins/#how-plugins-work and use it for future upgrades from the same source_account