Add support for ReservationGroup to bigqueryreservation

[jielin-bq]

This is to add support for ReservationGroup to bigqueryreservation by adding a reservation_group as a new resource as well as adding reservation_group as a new property to the existing resource reservation. See b/452350812 for more details.

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bigqueryreservation: added `reservation_group` field to `reservation` resource

`google_bigqueryreservation_reservation_group`

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 728 files changed, 2393 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 806 files changed, 2549 insertions(+), 4 deletions(-))
terraform-google-conversion: Diff ( 2 files changed, 125 insertions(+))
Open in Cloud Shell: Diff ( 4 files changed, 105 insertions(+))