Add auto-changelog generator

[ethanfrey]

We have no changelog in cw-plus. Which starts becoming an issue when more people use it and we are making changes besides bumping cosmwasm-std versions.

There are some tools that look at GitHub and make a template changelog based on closed issues and merged prs. Let's get one of these working on the repo and commit something.

We can work on a manual changelog later, but this is a quick step in the right direction.