Support policy tags in FieldSchema for column-level security

[syucream]

I hope to attach Data Catalog policy tags to fields for column-level security.
https://cloud.google.com/bigquery/docs/column-level-security-intro

It will just append policyTags field to FieldSchema like
googleapis/google-cloud-go@4c25d13

[syucream]

If it's good and nobody started working for that, I will create a patch. 💪

[plamut]

@syucream There is a plan to add column-level ACLs in the near future. But thanks nevertheless for the willingness to help!

cc: @shollyman