Changes PoolRef to a Value Type

[danehans]

Changes inferencemodel.spec.poolRef to a value type since it's required.

Fixes #111

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[kfswain]

Heya thanks for this! I just happened to see you make the issue and was fixing this along with some other issues.

There were some tests we had to update also that have a ref type, I included that. PR here: #114

If you're curious about our need for CI. I agree.
That PR here: kubernetes/test-infra#33987
Difficult to get review traction

[danehans]

@kfswain thanks for the heads-up. I searched the issue list and saw nothing related to #111 so I pushed this PR. I'll go ahead and close it since #114 fixes this issue. I'll do a review of #114 now.

[danehans]

Closed due to #112 (comment).