set default values for Instance Type for AWS CP and Worker nodes

[supershal]

InstanceType is a mandatory field in AWSMachineTemplate. Currently we set respective defaults CP: "m5.xlarge" and workers: "m5.2xlarge" in the default cluster class templates.

We should move it to the CAREN API so that the default values are discoverable to the CAREN Clients.

[supershal]

Please checkout d2iq-labs/cluster-api-runtime-extensions-nutanix@main...shalin/default_instance_type for an implementation that we discussed.

Currently AWS nodespec variable is shared between CP and worker node, that makes it difficult to set a single default in Json variable schema
This implementation sets default at the time of Discovery so that the CAREN client will have a default populated for respective CP and worker AWS node.

However the way the current API type is defined and JsonSchema is built it requires code changes in multiple places or need helper functions. I personally prefer some other solution than implemented in this PR.

Alternatives:

• Keep the defaults in the Default cluster class template (same as now) and add defaults to the variable description
• Create defaultInstanceType patches for AWS and Workers where it always replace value of instanceType with defaults if the AWSMachineTemnplate has value set to PLACEHOLDER. Leave current instanceType patch as it is. Document the defaults in the variable description
• Refactor API type to not use Generic types. i.e. use AWSClusterConfigSpec, AWSNodeSpec, instead of ClusterConfigSpec{"AWS": ...} and NodeSpec{"AWS": ...}
• mutating webhook
• any other ideas???
  cc: @dkoshkin , @faiq , @dlipovetsky , @jimmidyson

[supershal]

We had a discussion today. We decided to implement this with a mutating webhook which will add defaults to the cluster variables.