change `planetscale_database` `cluster_size` without replace

[joemiller]

The API does not expose the endpoint to change a planetscale_database's cluster_size so we currently need to do a replace (destroy + create) in the TF provider.

Ideal would be to add cluster_size to the PATCH database endpoint - https://api-docs.planetscale.com/reference/update_database_settings - to simplify the client code, but exposing the existing resizes endpoint should be manageable as well.