* Make `async:true` the default for `invoke` calls (#3750)
* Switch away from native grpc impl. (#3728)
* Remove usage of the 'deasync' library from @pulumi/pulumi. (#3752)
* Only retry as long as we get unavailable back. Anything else continues. (#3769)
* Handle all errors for now. (#3781)
* Do not assume --yes was present when using pulumi in non-interactive mode (#3793)
* Upgrade all paths for sdk and pkg to v2
* Backport C# invoke classes and other recent gen changes (#4288)
Adjust C# generation
* Replace IDeployment with a sealed class (#4318)
Replace IDeployment with a sealed class
* .NET: default to args subtype rather than Args.Empty (#4320)
* Adding system namespace for Dotnet code gen
This is required for using Obsolute attributes for deprecations
```
Iam/InstanceProfile.cs(142,10): error CS0246: The type or namespace name 'ObsoleteAttribute' could not be found (are you missing a using directive or an assembly reference?) [/Users/stack72/code/go/src/github.com/pulumi/pulumi-aws/sdk/dotnet/Pulumi.Aws.csproj]
Iam/InstanceProfile.cs(142,10): error CS0246: The type or namespace name 'Obsolete' could not be found (are you missing a using directive or an assembly reference?) [/Users/stack72/code/go/src/github.com/pulumi/pulumi-aws/sdk/dotnet/Pulumi.Aws.csproj]
```
* Fix the nullability of config type properties in C# codegen (#4379)
* Make Python StackReference test similar to others (with two steps)
* Include new Python StackReference integration test that uses multiple stacks
* Expose various life cycle methods for ProgramTester
These changes add a new resource to the Pulumi SDK,
`pulumi.StackReference`, that represents a reference to another stack.
This resource has an output property, `outputs`, that contains the
complete set of outputs for the referenced stack. The Pulumi account
performing the deployment that creates a `StackReference` must have
access to the referenced stack or the call will fail.
This resource is implemented by a builtin provider managed by the engine.
This provider will be used for any custom resources and invokes inside
the `pulumi:pulumi` module. Currently this provider supports only the
`pulumi:pulumi:StackReference` resource.
Fixes#109.
