feat(bigquery): support data governance tags on columns

[AxelThevenot]

Resolves #16693

Problem

BigQuery supports data governance tags for column-level access control, and dbt column properties cannot declare or apply them.

Solution

Add a data_governance_tags column property with one namespaced key and one value short name. When persist_docs.columns is enabled, dbt emits one ALTER TABLE that sets the tag on each supported column.

columns:
  - name: email
    data_governance_tags:
      key: my-project/data_class
      value: email

Supported:

• BigQuery tables
• Top-level, non-STRUCT columns
• One governance tag per column
• Setting and replacing a tag
  • key: <project-or-organization>/<tag-key>
  • value: <tag-value-short-name>

Not supported:

• Views and materialized views (BigQuery limitation)
• STRUCT or RECORD parent columns (BigQuery limitation)
• Nested STRUCT leaf fields
• Several tags on one column (BigQuery limitation)

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 or 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.

[codescene-delta-analysis]

Gates Failed
Enforce advisory code health rules (1 file with Large Method)

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Gates Passed
3 Quality Gates Passed

Reason for failure

Enforce advisory code health rules	Violations	Code Health Impact
utils.rs	1 advisory rule	5.01 → 5.01	Suppress

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Quality Gate Profile: Clean Code Collective
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[codescene-delta-analysis]

❌ Getting worse: Large Method
update_node_columns increases from 70 to 71 lines of code, threshold = 70

_Suppress