fix: use external Nutanix API types directly

[dkoshkin]

What problem does this PR solve?:
From what I can tell there is no longer a need to have these separate types.

These types get used by other projects where it generates a swagger spec. This change also fixes a swagger issue complaining about a circular reference since the types are named the same.

Which issue(s) this PR fixes:
Fixes #

How Has This Been Tested?:

Special notes for your reviewer:

[dlipovetsky]

The type aliases allow Go consumers to import only the CAREN package, so they are convenient.

Looks like go-swagger generates a self-referencing definition, instead of "resolving" the type alias:

Before the change:

 NutanixBootType:
    $ref: '#/definitions/NutanixBootType'
    title: NutanixBootType is an enumeration of different boot types.
  NutanixCategoryIdentifier:
    $ref: '#/definitions/NutanixCategoryIdentifier'

After the change:

  NutanixBootType:
    title: NutanixBootType is an enumeration of different boot types.
    type: string
    x-go-package: github.com/nutanix-cloud-native/cluster-api-runtime-extensions-nutanix/api/external/github.com/nutanix-cloud-native/cluster-api-provider-nutanix/api/v1beta1
  NutanixCategoryIdentifier:
    properties:
      key:
        description: |-
          key is the Key of category in PC.
        type: string
        x-go-name: Key
      value:
        description: |-
          value is the category value linked to the category key in PC
        type: string
        x-go-name: Value
    type: object
    x-go-package: github.com/nutanix-cloud-native/cluster-api-runtime-extensions-nutanix/api/external/github.com/nutanix-cloud-native/cluster-api-provider-nutanix/api/v1beta1

(Thanks to @dkoshkin for explaining the swagger issue)