AWSKMSAuthSpec API is suboptimal

Context:

To enable AWS KMS encryption we request a ref to a secret via API atm. Then we generate the requirements via cli. E.g

The secret is just a role_arn ref [1] to a role with the ability to assume [2] the policy [3] for the key

[1] https://github.com/openshift/hypershift/blob/main/api/fixtures/example.go#L150-L157
[2] https://github.com/openshift/hypershift/blob/a4fbc63f386561932e07253181d0fce788eb5ba4/cmd/infra/aws/iam.go#L677-L694
[3] https://github.com/openshift/hypershift/blob/a4fbc63f386561932e07253181d0fce788eb5ba4/cmd/infra/aws/iam.go#L333

https://coreos.slack.com/archives/C03TU14HLB0/p1672654814187489

DoD:

This is suboptimal api UX, we should only require the key arn in the API as input and let the backend controllers generate the above similar to what we did with AWSRolesRef.