[Feature] Allow policy tagging for snapshots in BigQuery

### Is this your first time submitting a feature request?

- [x] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [x] I have searched the existing issues, and I could not find an existing issue for this feature
- [x] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

Currently I'm adding policy tagging to my models, and I have snapshots within my pipeline. I can't add policy tagging to the snapshots however.

### Describe alternatives you've considered

Might be able to handle it on the bigquery side, but would then be undocumented.

### Who will this benefit?

Anyone looking for column level access in BigQuery within snapshots.

### Are you interested in contributing this feature?

I might be able to try?

### Anything else?

_No response_