Add support for processing_units to google_spanner_instance

[modular-magician]

Fixes: hashicorp/terraform-provider-google#9599

This will be a breaking change as the new field is mutually exclusive with num_nodes.

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• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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Release Note Template for Downstream PRs (will be copied)

spanner: `num_nodes` will become a required field in the next major release and a value has to be set on `google_spanner_instance`. It will also conflict with the new field `processing_units`.

spanner: added `processing_units` to `google_spanner_instance`.     

Derived from GoogleCloudPlatform/magic-modules#4993