convert contract to dict

[emmyoop]

resolves #7184

Description

Converts contract from a bool to it's own object

contract: bool = False
to

@dataclass
class Contract(dbtClassMixin, Replaceable):
    enforced: bool = False

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[gshank]

I have a question about the use of a dictionary here. In my PR (also up for review) I'm adding a "contract_checksum" as a separate field, but which I assume will go in this new contract "thing". Do we actually want it to be a dictionary or an object with fields, such as "enabled" and "checksum"? There are pros and cons for each...

[gshank]

Looks good, except for the location of the Contract class.

[gshank]

It doesn't look like the Contract class is used by anything in unparsed.py, so it ought to go in nodes.py. We should limit what nodes.py uses from unparsed.py as much as possible.

[emmyoop]

That creates circular imports with model_config. Instead of nodes.py I moved it into model_config.py. The only other thing defined in there is Hook but I think it still makes sense since it's not used in unparsed.py.