fix: add validation for rosa cluster subnets input

[scottd018]

The following logic needs to be added to the CRD CEL expression list for a ROSACluster resource:

• For a public multi-az cluster with provided subnets, 6 subnets should be provided (3 private, 3 public)
• For a public single-az cluster with provided subnets, 2 subnets should be provided (1 public, 1 private)
• For a private-link multi-az cluster with provided subnets, 3 subnets should be provided (3 private)
• For a private-link single-az cluster with provided subnets, 1 subnet should be provided (1 private)

Additionally, validation should happen as part of the reconciler logic to ensure anything that CRD CEL cannot validate does not make it too far into cluster creation:

• For a public cluster with provided subnets, there should be 2 subnets per AZ
• For a private-link cluster with provided subnets, there should be 1 subnet per AZ

The above needs to happen as part of the AWS client workflow (for now).

[paulczar]

mostly correct! the function that fetches the AZs for the subnets should always supply the number of AZs correctly (1 or 3) therefore the list should be uniquified, otherwise for a public cluster you'll have each AZ in the list twice because of the public AND private subnets.