Allow using open matching for open CapacityReservation

Description

What problem are you trying to solve? We want to acquire more g6 instance types in regions with low GPU counts (e.g ca-central-1) and can't add our existing instances to a capacity reservation. We want to retain the instances we have and claim any new available hardware as it becomes available. However, we discovered that all our existing instances have their capacity reservation set to "none".

We are struggling because we can not create a capacity reservation in the region with our existing instance counts because there is no g6 or g7 availability, and terminating the instances we do have would not necessary return to us or allow a capacity reservation to be opened successfully, and would impact our existing workloads.

Ideally we could use an open ODCR and use it to capture the existing hardware we have and then increase it's size over time to match any additional g6's in the region. However because of Karpenter's design this seems to not be possible. The Karpenter docs currently indicate that "open" is not a supported value. This seems to be an intentional design decision on the ODCR design that failed to consider the scenario around no existing capacity or when a targeted capacity reservation can not be created due to insufficient capacity.

So what we want is to be able to configure on an EC2NodeClass whether the default capacity reservation preference should be "none" or "open".

I also found https://github.com/aws/karpenter-provider-aws/issues/8176 but it was closed for inactivity.

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