Write contract upgrade strategy ADR for settlement hub

[mattdf]

The set of contracts in the settlement hub will not be static and never changed, added to or upgraded, hence a strategy needs to be decided on to perform protocol upgrades.

There are several options that can be implemented, and multiple could be supported for different deployments:

• Whitelist a multisig address on-chain that is deployed at genesis, which is the only one that can perform upgrades
• Use/implement cosmos governance module in optimint and leverage that to signal that a new client should be downloaded with a different contract set
• In the case of the settlement hub having multiple validators & a staking mechanic (either directly or via something like interchain security), the set of valid rollup contracts can be decided and whitelisted by validator majority