Add resource support for what if

[tgauth]

PR Summary

• fix #70
• example for resource participation, where whatif is implemented like set but with an additional switch arg:

{
    "$schema": "https://aka.ms/dsc/schemas/resource_manifest",
    // truncated
    "set": {
        "executable": "whatifexample",
        "args": [],
        "return": "state"
    },
    "whatIf": {
        "executable": "whatifexample",
        "args": [
            "--what-if"
        ],
        "return": "state"
    },
    // truncated
}

• example output from resource that implements whatif, format matches set:

PR Context

• add resource support for config set --what-if

[SteveL-MSFT]

Should there be a separate schema for what resources return for what-if that aligns with how the engine synthesizes it? Perhaps with addition of reason field that is open text that the resource can fill in

[anmenaga]

@tgauth please open an issue to add 'whatif' support to PS Adapter.

[SteveL-MSFT]

Based on the review meeting:

• Add new whatIf section as a top-level operation in the resource manifest which only applies to set, all other changes in the code will flow from this

[SteveL-MSFT]

@tgauth can you provide example of what-if output where a resource participates?