[RFC] Extend one enviroment with values from another

**Is your feature request related to a problem? Please describe.**
I find it annoying that i have to duplicate environmental settings or secrets, and i want to be able to daisy chain settings together and extend other environments

**Describe the solution you'd like**
If i have multiple environments described in my settings or secrets.yaml files, i want to be able to extend other envirnments, via a key or similar.
```
default:
..

development:
key: ab

local:
_extends: development
env: local
```
here settings.local.key should be "ab"