feat: [BigQueryReservation] Add Reservation.max_slots field to Reservation proto, indicating the total max number of slots this reservation can use up to (googleapis#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: https://github.com/googleapis/googleapis-gen/commit/9fc20ac07dd1d4e1c31c5b56bc10c9fd33c55f90
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