add set/unset_query_tag to the dbt macro namespace

[robscriva]

resolves #133

Description

There is no way of overriding the set_query_tag and unset_query_tag macros to create a custom query tag within an external package. This is because these macros aren't part of the dbt namespace and therefore cannot be overridden with dispatching precedence in the project. This change adds both macros to the dbt macro namespace so they can be overridden externally.

Discussion here: https://getdbt.slack.com/archives/C37J8BQEL/p1633519489312300?thread_ts=1633377998.228400&cid=C37J8BQEL

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-snowflake next" section.

[jtcohen6]

@robscriva Looking good! One comment from me. Otherwise, this will be good to ship in v1.2.

[jtcohen6]

Not sure what's going on with those test failures, will need to take a more detailed look

[robscriva]

Just a coincidence after I swapped to use the snowflake__ prefix right? 😂

[jtcohen6]

@robscriva Definitely a coincidence. We're seeing those tests fail in the main branch as well, on nightly runs (e.g. this one). Sorry that your PR is being held up in the meantime.

[jtcohen6]

@robscriva Looks great, thank you!

I just added a contributor entry. We can merge as soon as the tests pass.