Add computed to node_config.kubelet_config & node_pool_auto_config.node_kubelet_config

[modular-magician]

Fixes #19792

Support for the node_config.kubelet_config.insecure_kubelet_readonly_port_enabled field was added in GoogleCloudPlatform/magic-modules#11573 and was marked Computed. node_config itself is also Computed, however node_config.kubelet_config is not.

This means that:

• A configuration that does not specify node_config is not affected
• A configuration that specifies node_config but not node_config.kubelet_config is affected
• A configuration that specifies node_config.kubelet_config is not affected.

This caused numerous failing tests for us, however, not all the tests we would have expected to fail based on those criteria.I haven't determined another trigger condition so I suspect it's a partial rollout of a serialisation change. This was confirmed as a rollout in #19792 (comment).

kubelet_config appears in multiple other locations, node_pool_defaults.node_config_defaults.insecure_kubelet_readonly_port_enabled and node_pool_auto_config.node_kubelet_config.insecure_kubelet_readonly_port_enabled

• node_pool_defaults.node_config_defaults has the same problem, as it is not Computed while node_pool_defaults and insecure_kubelet_readonly_port_enabled are. However, sending just node_pool_defaults {} already triggers the same issue with another field (logging_variant). Ideally we'd make node_config_defaults Required, but I'm not sure about the impact on autopilot. I'll file a bug.
• I don't expect a default value to be set for node_pool_auto_config.node_kubelet_config.insecure_kubelet_readonly_port_enabled but I've set node_pool_auto_config.node_kubelet_config to Computed anyways since it could exhibit the same issue and I would not want to have to deal with the same problem again. I may cut the change if I can confirm it will never get a default (but I'm not guaranteed / unlikely to find that answer before shipping the change)

The update part of the original issue (Error: googleapi: Error 400: At least one of ['node_version', <snip>, 'max_run_duration'] must be specified.) is because we used ForceSendFields with a nil KubeletConfig value, which does nothing. We had to set KubeletConfig to &container.KubeletConfig{} in that block. NullFields would be correct for a patch in theory, but this is a PUT / customish update and the JSON null still triggers the API error.

After this change we can no longer trigger that specific update message. I'll still clean up that callsite / any similarly structured ones to ensure we send an empty struct properly if we do hit them, but in a separate change.

Release Note Template for Downstream PRs (will be copied)

container: fixed a diff triggered by a new API-side default value for `node_config.0.kubelet_config.0.insecure_kubelet_readonly_port_enabled`. Terraform will now accept server-specified values for `node_config.0.kubelet_config` when it is not defined in configuration and will not detect drift. Note that this means that removing the value from configuration will now preserve old settings instead of reverting the old settings.

container: `google_container_cluster` will now accept server-specified values for `node_pool_auto_config.0.node_kubelet_config` when it is not defined in configuration and will not detect drift. Note that this means that removing the value from configuration will now preserve old settings instead of reverting the old settings.

Derived from GoogleCloudPlatform/magic-modules#11971

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