Add a changelog entry check for pull requests

[mikealfare]

resolves #57

Problem

We don't have a check for changelog entries on pull requests, which could lead to changes not being captured in the release changelog.

Solution

Add the check that we have in other adapter repos.

Checklist

• I have read the contributing guide and understand what's expected of me
• 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
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-postgres contributing guide.