Allow to use a costum field as check-cols updated_at

[PJGaetan]

resolves #1844

Description

Allows the use of updated_at field while using check strategy in a snapshot.
The timestamp used to update dbt_valid_from will come from the costum field instead of the default current timestamp.

I have added a test to ensure that rows are invalidated with the right timestamp. Not sure it is in the right place to add the test but I didn't know how to pick up relation_b otherwise.

Checklist

• 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 updated the CHANGELOG.md and added information about my change to the "dbt next" section.

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: PierreJustin Gaetan.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
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[jtcohen6]

@PJGaetan This looks great to me! Nice work on the test.

I left a small suggestion on the changelog. Could you also:

• Pull in develop
• Move your changelog note up to v0.20.0 (rather than v0.20.0b1)
• Add yourself to the list of Contributors

[jtcohen6]

Just one small suggestion on the change wording, to make it clear what the current/default behavior is:

Suggested change

	Add optional `updated_at` config parameter to invalidate rows with the inputed column timestamp in snapshot using `check` strategy. ([#1844](https:/fishtown-analytics/dbt/issues/1844), [#3376](https:/fishtown-analytics/dbt/pull/3376))
	- Support optional `updated_at` config parameter with `check` strategy snapshots. If not supplied, will use current timestamp (default). ([#1844](https:/fishtown-analytics/dbt/issues/1844), [#3376](https:/fishtown-analytics/dbt/pull/3376))

[PJGaetan]

Definitely fells better this way

[jtcohen6]

Thanks for the contribution @PJGaetan!