Fixes#2277.
Adds a new ignoreChanges resource option that allows specifying a list of property names whose values will be ignored during updates. The property values will be used for Create, but will be ignored for purposes of updates, and as a result also cannot trigger replacements.
This is a feature of the Pulumi engine, not of the resource providers, so no new logic is needed in providers to support this feature. Instead, the engine simply replaces the values of input properties in the goal state with old inputs for properties marked as ignoreChanges.
Currently, only top level properties may be specified in ignoreChanges. In the future, this could be extended to support paths to nested properties (including into array elements) with a JSONPath/JMESPath syntax.
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