dynamically retrieve instance types

[bwagner5]

Issue #, if available:
#286

Description of changes:

• Fetch instance types dynamically w/ ec2.DescribeInstanceTypes and transforms to nodeCapacity information.
• Ran go mod tidy which updated some go.sum stuff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ellistarn]

Do we want to error here? Is debug better? IIUC, this only errors if the theoretical node is full, which might happen multiple times in simulation.

[JacobGabrielson]

I'd probably not consider it an error even - although it should never even happen, if there were some really extreme instance type in the future where perhaps it did trigger, it means the instance type cannot handle kubelet overhead at all, so we shouldn't consider using it anyway - maybe make this a log info ("excluding ...")?

[bwagner5]

I think that makes sense for info level. I just brought this in from a rebase, so not sure if there was a reason for it being error.

[ellistarn]

We're really judicious about Info level -- we can talk about the ideology here, maybe debug?

[bwagner5]

I don't have a super strong opinion, so I can go either way. I think Jacob is right that it shouldn't ever happen the way the kubelet overhead is being calculated right now since it's a scaled a percentage. But I suppose if there was a weird instance type that was released, it could spam everyone using karpenter if it's at info level, so I'm fine with debug.

[ellistarn]

Nice work. More or less LGTM, a couple nits.

[JacobGabrielson]

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