[fix] Use rendered query comment for job labels

[kodaho]

resolves #863

Problem

A regression appears in the last release of dbt-bigquery. A user can previously add labels to a BigQuery job thanks to a macro returning a mapping. In the version 1.6.0, this was no longer possible even if the project is set to activate this feature.

Solution

The proposed solution is to use, instead of self.profile.query_comment (which is the not-rendered Jinja macro), self.query_header.comment.query_comment (the JSON rendered query comments).

I added some safeguards to make sure that self.query_header.comment.query_comment is not null but during the runtime, it should never be the case because of the following succession of functions and methods calls during a run command:

1. The query labels are sent with a run command, command decorated with the decorator requires.manifest.
2. The class method ManifestLoader.get_full_manifest is called.
3. The method ManifestLoader().save_macros_to_adapter is called.
4. ManifestLoader().macro_hook is called, knowing that macro_hook = adapter.connections.set_query_header.
5. Calling Connection().set_query_header makes sure that self.query_header is not null.

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

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[McKnight-42]

@kodaho thank you for contributing this was working on something locally. It was a fantastic pull in updating the test as well.