* Revise host mode.
The current implementation of host mode uses a `pulumi host` command and
an ad-hoc communication protocol between the engine and client to
connect a language host after the host has begun listening. The most
significant disadvantages of this approach are the communication
protocol (which currently requires the use of stdout), the host-specific
command, and the difficulty of accommodating the typical program-bound
lifetime for an update.
These changes reimplement host mode by adding engine support for
connecting to an existing language runtime service rather than launching
a plugin. This capability is provided via an engine-specific language
runtime, `client`, which accepts the address of the existing languge
runtime service as a runtime option. The CLI exposes this runtime via
the `--client` flag to the `up` and `preview` commands, which similarly
accepts the address of an existing language runtime service as an
argument. These changes also adjust the automation API to consume the
new host mode implementation.
* 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)