Problem: no easy way to upgrade contract bytecode

We can take advantage of the builtin gov process to do contract upgrade, implement a contract upgrade proposal type, proposer submit the contract address and bytecode, after proposal passed, the contract is upgraded.

## Advantages

- contract address don't change
- don't need any dev effort in contract level

## Disadvantage

- need the whole chain gov process to upgrade an individual contract, so probably only works for important contracts.

The issue names no files, tests, or entry points. Start by locating the built-in governance proposal handling and the existing contract upgrade mechanism; done means a proposal can carry a contract address and bytecode and, after approval, upgrade the contract without changing its address.

Written by the indexing model from the issue text.