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!

[b-per]

Not entirely related to meshify but this is an interesting statement about contracts + schema evolution.

Based on what you observed, if a contracted incremental model has 4 columns, a, b, c, d and if at some point one column is dropped, the model will run fine until we do a --full-refresh which will then make it fail.

This is going to create a few headaches when going through CI/CD, no?