handle `on_schema_change` for incrementals with contracts

[dave-connors-3]

Incremental models with contracts must have the on_schema_change config set to append_new_columns. This will need to be a conditional piece of logic in the add-contract command.

Options

1. add it to the model yml entry blindly
2. be more creative and find where the config is set from and override apropriately.

1 feels good short ter, 2 feels right long term!