Allow retrieving and updating database cluster configuration

**What is the problem this feature would solve? Please describe.**

In the DO API docs, I found that it is possible to [retrieve database cluster configuration](https://docs.digitalocean.com/reference/api/api-reference/#operation/databases_get_config) and also to [update it](https://docs.digitalocean.com/reference/api/api-reference/#operation/databases_patch_config). However, using the API directly is somewhat cumbersome.

**Describe the solution you'd like**
It would be great to have a `doctl` to do this, e.g. `doctl databases configuration get `. The output would probably be JSON, or maybe could be formatted with a flag. For setting the input should probably also be JSON?