Add whatif

[tgauth]

PR Summary

• add what-if flag to config set with synthetic test as a starting point, until resources can eventually implement what-if
• if a resource does not implement its own pre-test, it returns the synthetic test output
• if a resource implements its own pre-test then it errors, since the synthetic test output seems less valuable/accurate in this scenario? open to input here
• delete with what-if is not a supported scenario

PR Context

• fix Handle WhatIf #70
• example of config set -w when actual state already matches desired state (first test in dsc_whatif.tests.ps1):
  
• example of config set -w when actual state does not match desired state (second test in dsc_whatif.tests.ps1):
  

[anmenaga]

Please add 2 examples of output to PR description for:

1. running a resource operation in what-if mode;
2. running a configuration operation in what-if mode.

[SteveL-MSFT]

@tgauth can you address the merge conflicts?

[tgauth]

@tgauth can you address the merge conflicts?

done!

[SteveL-MSFT]

Some minor feedback, otherwise looks pretty good