Links to Vault subschemas

[lena-larionova]

• Add a oneOf to the vault schema's name parameter in the API specs

          name:
          description: The name of the Vault that's going to be added. The Vault implementation must be installed in every Kong instance.
          type: string
          oneOf:
            - env
            - aws
            - azure
            - gcp
            - hcv

• Add a notOneOf (not sure about the name here) to the prefix parameter

          prefix:
          description: |
            The unique prefix (or identifier) for this Vault configuration. 
            The prefix is used to load the right Vault configuration and implementation when referencing secrets with the other entities.
          type: string
          notOneOf:
            - env
            - aws
            - azure
            - gcp
            - hcv

• Link from the Vault config object entry to wherever we document the subschemas

[lena-larionova]

Depends on #346