Model contract configs

[dave-connors-3]

This PR adds the configs for the upcoming 1.5 dbt core concepts for model contracts

you can:

For projects:

• define groups of models in _resource.yml files.

For models

• define access - whether the model is public/private/protected
• define contract - whether the model is subject to a model contract
• define group - assign a model to a group

For columns:

• define constraint - add a database constraint to the model
• define constraint_check add a SQL check on the column

For metrics:

• define groups in config -- sneaking in a config object, as metrics will be able to be grouped -- adding the other two metric configs while I was in there (can split this out if we'd prefer to sequence these more thoughtfully)