helper/schema: ResourceDiff ForceNew attribute correctness

[vancluever]

A couple of bugs have been discovered in ResourceDiff.ForceNew:

• NewRemoved is not preserved when a diff for a key is already present.
  This is because the second diff that happens after customization
  performs a second getChange on not just state and config, but also on
  the pre-existing diff. This results in Exists == true, meaning nil is
  never returned as a new value.
• ForceNew was doing the work of adding the key to the list of changed
  keys by doing a full SetNew on the existing value. This has a side
  effect of fetching zero values from what were otherwise undefined values
  and creating diffs for these values where there should not have been
  (example: "" => "0").

This update fixes these scenarios by:

• Adding a new private function to check the existing diff for
  NewRemoved keys. This is included in the check on new values in
  diffChange.
• Keys that have been flagged as ForceNew (or parent keys of lists and
  sets that have been flagged as ForceNew) are now maintained in a
  separate map. UpdatedKeys now returns the results of both of these maps,
  but otherwise these keys are ignored by ResourceDiff.
• Pursuant the above, values are no longer pushed into the newDiff
  writer by ForceNew. This prevents the zero value problem, and makes for
  a cleaner implementation where the provider has to "manually" SetNew to
  update the appropriate values in the writer. It also prevents
  non-computed keys from winding up in the diff, which ResourceDiff
  normally blocks by design.

There are also a couple of tests for cases that should never come up
right now involving Optional/Computed values and NewRemoved, for which
explanations are given in annotations of each test. These are here to
guard against future regressions.

[vancluever]

Thanks @jbardin!

[ghost]

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