Add support for creating bigquery labels from meta

[physcat]

Introduces a new feature flag, labels_from_meta, that allows adding labels from the meta config into labels. If enabled, meta labels are included alongside existing labels, with labels taking priority in case of conflicts.
Default behavior remains unchanged unless the flag is explicitly set.

resolves #889
docs dbt-labs/docs.getdbt.com/#

Problem

Currently, users can add BigQuery labels through the explicit labels configuration, but there's no way to leverage existing metadata in the meta field as labels. This creates duplicate work when users want to expose the same information in both places.

Solution

Implemented a new labels_from_meta configuration flag that, when enabled, merges items from the meta section with the labels configuration. This allows users to maintain a single source of information while still controlling which metadata is exposed as BigQuery labels.

Key implementation details:

• Default behavior remains unchanged (backward compatible)
• When enabled, meta values are included alongside existing labels
• In case of conflicts, labels entries take precedence

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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[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change in the dbt-bigquery package. For details on how to document a change, see the Contributing Guide.