feat: [BigQueryReservation] Add a new field soft_failover_start_time in the existing replication_status in .google.cloud.bigquery.reservation.v1.Reservation to provide visibility into the state of ongoing soft failover operations on the reservation (#8554)

* feat: Add a new field `soft_failover_start_time` in the existing `replication_status` in `.google.cloud.bigquery.reservation.v1.Reservation` to provide visibility into the state of ongoing soft failover operations on the reservation
feat: Add a new field `failover_mode` to `.google.cloud.bigquery.reservation.v1.FailoverReservationRequest` that allows users to choose between the HARD or SOFT failover modes when they initiate a failover operation on a reservation

PiperOrigin-RevId: 802187284

Source-Link: googleapis/googleapis@afe86f3

Source-Link: googleapis/googleapis-gen@df2db68
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