feat: Add support for specifying stack type for clusters. This will a…

…llow clusters to be created as dual stack or toggled between IPV4 and dual stack

Clients can now configure dual stack clusters by specifying a stack type of IPV4_IPV6 during cluster creation, or with an update command. When used with a cluster creation command that creates a new subnet, the ipv6_access_type field can be used to specify whether the subnet has internal or external IPv6 access. When used with a cluster update command, the subnet must already be dual stack.

PiperOrigin-RevId: 495438580

Source-Link: googleapis/googleapis@483c1ee

Source-Link: googleapis/googleapis-gen@20bdefc
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