What is the problem this feature would solve? Please describe.
doctl aims to be useful in scripting. One way it does this is by providing a --wait flag for long running, async operations to block until they are complete rather than making the user implement polling in their own script. Some examples include:

• doctl database create --wait
• doctl compute droplet create --wait
• doctl kubernetes cluster create --wait
• doctl apps create --wait

Currently the doctl databases resize command does not have a --wait flag.

Describe the solution you'd like
Add a --wait flag to the doctl databases resize command. It should poll the /v2/databases/$DATABASE_ID/resize, blocking until the database cluster's status attribute moves from resizing to online.

Additional context
Following the code path starting here demonstrates how doctl database create's --wait flag is implemented: