Feature gap: Add missed fields for Reservation resource

[modular-magician]

This PR provides below fields to providers:

• specificReservation.instanceProperties.maintenanceInterval - beta
• specificReservation.sourceInstanceTemplate - ga
• deleteAtTime - ga
• deleteAfterDuration.seconds - ga
• deleteAfterDuration.nanos - ga
• shareSettings.projects - beta
• reservationSharingPolicy.serviceShareType - ga
• enableEmergentMaintenance - beta

Additional information:

• deleteAfterDuration is ignore_read field, because it's used to set deleteAtTime field. deleteAfterDuration is not returned by API.
• enableEmergentMaintenance is ignore_read field, because it's not returned by API

Release Note Template for Downstream PRs (will be copied)

See Write release notes for guidance.

compute: added `specific_reservation.0.instance_properties.0.maintenance_interval`, `share_settings.0.projects` and `enable_emergent_maintenance` fields to `google_compute_reservation` resource (beta)

compute: added `specific_reservation.0.source_instance_template`, `delete_at_time`, `delete_after_duration.0.seconds`, `delete_after_duration.0.nanos` and `reservation_sharing_policy.0.service_share_type` fields to `google_compute_reservation` resource (ga)

Derived from GoogleCloudPlatform/magic-modules#14181