refactoring: Introduce Reservation dataclass

[jamOne-]

Description

Main goal: introduce Reservation data class containing all information XPK reads from the GCE Reservation resource.

Other changes:

• One entry-point function for fetching the reservations by the ReservationLink. Easily cache-able.
• TBH XPK currently don't require acceleratorInfo, nor the real Reservation capacity info (count, inUseCount), but I'll need it soon.
• Behavioral change: reservations without READY status are treated as incorrect, previously we didn't check that.

Reservations API reference: https://docs.cloud.google.com/compute/docs/reference/rest/v1/reservations.

Issue

Part of b/477114712.

I need this change to focus on business logic in the better reservations handling logic, where I'll ensure Reservation capacities and machines types are correct.

Testing

• unit tests,
• goldens,
• manually tested with Super-slicing reservation.