feat: [BigQueryReservation] Add Reservation.max_slots field to Reservation proto, indicating the total max number of slots this reservation can use up to (#8525)

* feat: Add Reservation.max_slots field to Reservation proto, indicating the total max number of slots this reservation can use up to

feat: Add Reservation.scaling_mode field and its corresponding enum message ScalingMode. This field should be used together with Reservation.max_slots
PiperOrigin-RevId: 795606766

Source-Link: googleapis/googleapis@6388d14

Source-Link: googleapis/googleapis-gen@9fc20ac
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