bigqueryreservation: add principal field to ReservationAssignment

[fuzmish]

Summary

This is a re-submission of #17080, which was closed without merge.

The purpose is to support User-specific assignments by adding the principal field to google_bigquery_reservation_assignment. As of June 30, 2026, the feature has been officially released as Public Preview, and the field is now explicitly present in the v1 REST API discovery document. Based on this, we have set this to GA field (not min_version: beta).

Changes:

• Add principal field to ReservationAssignment resource. Description is taken verbatim from the v1 discovery document.
• Add new example bigquery_reservation_assignment_full_with_principal (with corresponding test) to cover the principal field. A separate example is used rather than modifying the existing _full example, as principal-based assignment is a distinct usage pattern.

Testing

The provider was built locally from this branch and tested with dev_overrides. Resource creation with principal set to a service account principal was verified to succeed end-to-end.

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bigqueryreservation: added `principal` field to `google_bigquery_reservation_assignment` resource

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