Update dbt-core to 1.5.9 and implement support for model contracts

[mwallace582]

resolves #145

Description

I've followed the instructions published for updating to dbt-core 1.5 and implemented support for model contracts. I targeted 1.5.9 given that it was the latest and greatest version of 1.5.

I was able to get all of the new test classes to pass, and model contracts are working as expected for my dbt models.

There's some trickiness with the difference between types that MySQL allows in schemas, and the types that can be passed into calls to CAST(). I've handed some of the obvious cases, but it's possible that there are gaps with more obscure types.

Not all of my changes are strictly necessary (such as the upgrade to mysql-connector-python). I'd be happy to take any and all feedback.

Checklist

• 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 updated the CHANGELOG.md with information about my change

[cknowles]

@dbeatty10 is there some way I could help get this PR merged?

[mwallace582]

I'm closing this MR in favor of #163 which contains the same work.