[tgrade-validator-voting] Add MigrateContracts proposal

[maurolacy]

Add a MigrateContracts proposal that takes an old_code_id and a new_code_id, and automatically migrates all the contract instances associated to the old_code_id to the new_code_id.

This is useful to migrate a bunch of instances of the contract at once. By example, if there are dozens of tokens (cw20-base contract), trusted tokens (trusted-token contract), and tfi pairs (tfi-pair contract), and one of those contracts has a bug or a new feature that requires a migration, it will be handy to have this proposal type, to migrate all the instances of said contract at once.

This assumes the migrate admin of those tokens is the validator voting contract. Which is the case for tfi-pair already.