These changes add two new methods to the provider interface and extend
the results of three others.
The new methods are `CheckConfig` and `DiffConfig`, which fill out the
set of methods required for a complete implementation of the
first-class provider design. Though these methods are optional for
backwards compatibility, they should be implemented by all future
providers for the best possible user experience.
The adjusted result types are `DiffResponse`, `ReadResponse`, and
`ErrorResourceInitFailed`. The first has been updated to include a list
of the properties that changed (if any). The latter two now include
an estimated set of inputs for the resource as well as the resource's
state. Together, these three changes enable the engine to determine the
set of inputs that should be specified by a user in order to match those
that describe the resource's current state.
This contributes to #2453, #1662, #1635, and #1718.
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