If a data source is storing a value that doesn't comply precisely with
the schema, it will now show up as a perpetual diff during plan.
Since we can easily detect if there is no resulting change from the
stored value, rather than presenting a planned read each time, we can
change the plan to a NoOp and log the incongruity as a warning.
The new data source planning logic no longer needs a separate action,
and the apply status can be determined from whether the After value is
complete or not.
The state was not being set, so the change was not evaluated correctly
for dependant resources.
Remove use of cty.NilVal in readDataSource, only one place was using it,
so the code could just be moved out.
Fix a bunch of places where warnings would be lost.
Rather than re-read the data source during every plan cycle, apply the
config to the prior state, and skip reading if there is no change.
Remove the TODOs, as we're going to accept that data-only changes will
still not be plan-able for the time being.
Fix the null data source test resource, as it had no computed fields at
all, even the id.
The logic for refresh, plan and apply are all subtly different, so
rather than trying to manage that complex flow through a giant 300 line
method, break it up somewhat into 3 different types that can share the
types and a few helpers.
