add support for advanced mysql config get and patch operations

[gangasingh01]

The Aim of this PR is to add support for ADVANCED_MYSQL configuration patch and get operations

doctl databases configuration get <CLUSTER_UUID> \
  --engine advanced_mysql \
  -o json \
  --api-url=https://api.digitalocean.com \
  --access-token <ACCESS_TOKEN>

doctl databases configuration update <CLUSTER_UUID> \
  --engine advanced_mysql \
  --config-json '{"mysql_parameters":{"max_connections":"200"}}' \
  --api-url=https://api.digitalocean.com \
  --access-token  <ACCESS_TOKEN>

[anup-deka]

LGTM