feat: API for encryption at-rest

[supershal]

What problem does this PR solve?:

• CAREN API for encryption at-rest

topology:
    variables:
      - name: clusterConfig
        value:
          encryption:
            providers: 
              - aescbc: {}

• Define API schema
• Implement mutator handler
• unit tests
• e2e tests
• documentation

Which issue(s) this PR fixes:
Fixes # https://jira.nutanix.com/browse/D2IQ-100555

How Has This Been Tested?:

Special notes for your reviewer:

[dkoshkin]

https://github.com/kubernetes/kubernetes/blob/723c5e99d80a22abc9faea2f8b36b39bf6987a23/staging/src/k8s.io/apiserver/pkg/apis/apiserver/types_encryption.go#L84-L101

      - name: clusterConfig
        value:
          encryption:
            providers: 
              - aescbc: {}
              - kms: {}

type Encryption struct {
	Providers []EncryptionProviderConfiguration `json:"providers"`
}

type ProviderConfiguration struct {
  	// optional
	AESCBC *AESConfiguration
  	// optional
	KMS *KMSConfiguration // not actually needed right now, but here for an example
}

type AESConfiguration struct {}
type KMSConfiguration struct {}

[dkoshkin]

Looks good, just some minor comments.