container: fix in-place updates for node_config.containerd_config

[modular-magician]

It appears that in-place updates were never implemented for node_config.containerd_config, when contained within either google_container_cluster or google_container_node_pool

Fixes #19056
Related to #19225 (already closed / fixed)

The actual implementation is pretty simple, and is basically copy / paste from other fields.

The adjustments to existing tests are a little trickier to parse out.

Basically, the existing tests for both google_container_cluster and google_container_node_pool mostly covered node_config_defaults (maybe because that's the only one updates worked for), and didn't cover updates in node_config. In at least one case, the same test was used for all of node_config_defaults, node_pool.node_config, and node_config, and I've mostly preserved that, just adjusting the order. Note that in at least one case, that will likely require recreating the entire cluster and / or its node-pools; I've moved that test (named node_pool {} block in google_container_cluster) to be the last test, and attempted to add some comments.

Another approach would be to split out the tests of node_config_defaults, node_pool.node_config, and node_config.

Release Note Template for Downstream PRs (will be copied)

container: fixed in-place updates for `node_config.containerd_config` in `google_container_cluster` and `google_container_node_pool`

Derived from GoogleCloudPlatform/magic-modules#12135