* 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.
