Feature request: Extra deployment config

My applications need some "extra" configuration settings that apply on the deployment group level.

So, it would be great if I could have a way to put this data into the deployment group, similar to how "user data" works when starting EC2 instances.

If this data were available as a file somewhere near the deployment directory (like `/opt/codedeploy-agent/deployment-root/{deployment-group-id}/{deployment-id}/extra-config`), that'd be all I need :+1:.

**Side note**

My application is kept in a GitHub repository. So technically, I could add a file (say `.deployment-config`) into this repo and keep the "extra" data there. To support multiple deployment groups, that file would need to be keyed or have "sections".

However, since the data makes sense only within a particular deployment group, this seems to be more cumbersome. Over time, the file in the repo would still contain a history of deployment configurations for deployment groups long gone. Also, when people create different deployments for feature branches, unneccessary merging conflicts will occur sooner or later. This all could be avoided with the dedicated "extra config" data field.