mod: added support for autoscaling

[gangasingh01]

The aim of this PR is to add support for storage autoscale options for databases using doctl

Attaching the test evidences for the get and update calls for the same

GET storage autoscale details

./doctl databases storage-autoscale get \ 
  <cluster_uuid> \
  --api-url=https://api.digitalocean.com \
  --access-token <token>

UPDATE storage autoscale details

./doctl databases storage-autoscale update \
   <cluster_uuid>\
  --enabled true \
  --threshold-percent 60 \
  --increment-gib 40 \
  --api-url=https://api.digitalocean.com \
  --access-token <token>