feat: created reservation samples for update and create

[BigBlackWolf]

Description

• Sample for creating reservation manually, without template
• Sample for updating number of VMs in reservation
• Updated getReservation sample
• Moved previous set of tests into a separate case

Checklist

• [ ] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [ ] Tests pass: go test -v ./.. (see Testing)
• [ ] Code formatted: gofmt (see Formatting)
• [ ] Vetting pass: go vet (see Formatting)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

- [compute/reservations/create_base_reservation.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/00a3d5bd95ad23a7025787071de357020515cb8e/compute/reservations/create_base_reservation.go#L17), tag `compute_reservation_create` - [compute/reservations/update_reservation_vms.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/00a3d5bd95ad23a7025787071de357020515cb8e/compute/reservations/update_reservation_vms.go#L17), tag `compute_reservation_vms_update`

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