[Feature] Support using custom connection for BigQuery Iceberg tables

[yingying-chen-cko]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Currently for Bigquery Iceberg tables, it only supports using the default connection: https://github.com/dbt-labs/dbt-adapters/blob/main/dbt-bigquery/src/dbt/include/bigquery/macros/adapters.sql#L31

It'd would be great to allow specifying custom connection name.

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response