Allow WalkResources to walk all resources (of all types) if resourceType not specified

[eczy-tempus]

Modifies the WalkResources function such that if resourceType is passed the string zero-value "", the function walks over all resources of all types.

[wata727]

Hi @eczy-tempus, thank you for your contribution.
Now I'm working on a major update to the SDK and the current APIs, including WalkResources, will change significantly #135

If this change isn't urgently needed, I recommend waiting for a new version to be released. If you would like to take advantage of this change immediately, please let me know. Thanks.

[eczy-tempus]

@wata727 This change isn't urgently needed. I've already implemented an external function that achieves the same functionality. The purpose of this PR is to add the ability to access all resources (regardless of type) to this library in case others have similar requirements.

It appears that in #135 this would be possible like so:

providers, _ := runner.GetModuleContent(&hclext.BodySchema{
    Blocks: []hclext.BlockSchema{
        {
            Type: "resource",
            ...
        },
    },
}, nil)

Is this correct? If so, then this functionality will have already been implemented, and I can close this PR.

[wata727]

Yes, in the new SDK version, that example works correctly.
I currently investigating compatibility in edge cases. Please wait a moment.

[eczy-tempus]

@wata727 Closing this PR since this functionality will be covered in an upcoming release. Thank you for your hard work in maintaining this tool!