Support stack transformations

[t0yv0]

Hello!

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Issue details

This is a feature supported in other language SDKs but missing from the Java SDK.

https://www.pulumi.com/docs/intro/concepts/resources/options/transformations/#stack-transformations

Affected area/feature

[pawelprazak]

API probably would look like this:

ctx.registerStackTransformation(Function<ResourceTransformationArgs, ResourceTransformationResult>)

[pawelprazak]

Looks like we have this, but was hidden with the Stack class

[pawelprazak]

I've had a look at it, but I did too many refactorings. I think we need to do #488 first.

[pawelprazak]

After consulting with @t0yv0 we'll get back to it after preview.

[pawelprazak]

Unfortunately, setting transformations on the context (like in Go) is not possible with the current implementation of Resource.

Since Stack inherits Resource initialization logic, the transformations are registered in the constructor.
Also the Stack needs to exist before any user code is executed (including setting the transformations on the context).

Alternative APIs:

dotnet inspired (very easy to implement)

Pulumi.run(ctx -> { ... }, StackOptions.builder().transformations(...).build())

or a "fluent" approach (new public interface will be required for the withStackTransformations to have something to return

Pulumi
    .withOptions(StackOptions.builder().transformations(...).build())
    .run(ctx -> {...})

I personally don't have strong preference (slight preference for the second one, since it is consistent with what I plan for PulumiTest and PulumiAuto APIs).