Move size reservations to a dedicated table.

[Gerrit91]

Now that we would like to offer size reservations directly to customers, it appears that managing all the size reservations inside a slice of a size is quite complicated.

For just operator-usage this is fine, but if you want to select specific reservation entities, it's quite difficult for individual projects that do not have access to the entire reservation slice. It leads to CLI commands like machine-reservation describe <partition> <project> <size> in order to correctly identify a reservation (see GenericCLI PR here). But then it's still quite hard to select the proper reservation because you need to introduce implicit conventions for uniqueness. What it really lacks is an identifier.

In order to solve this, we should just treat the entity like any other and put it into a dedicated table. We can then also just have regular CRUD endpoints for the entity, which makes it easier to handle.

This is unfortunately a breaking change because the size reservations move away from the size entity. But for now only operators have been using this feature, so a breaking change should not have such a big impact on this resource.

Breaking Change

Size reservations were moved into a dedicated table and are not directly part of the size entity anymore. In this context, the API has also been slightly changed. If you used size reservations in your deployment, please adopt to the new deployment scheme. Existing reservations are migrated into the new table and do not get lost.