This change adds a GetRequiredPlugins RPC method to the language
host, enabling us to query it for its list of plugin requirements.
This is language-specific because it requires looking at the set
of dependencies (e.g., package.json files).
It also adds a call up front during any update/preview operation
to compute the set of plugins and require that they are present.
These plugins are populated in the cache and will be used for all
subsequent plugin-related operations during the engine's activity.
We now cache the language plugins, so that we may load them
eagerly too, which we never did previously due to the fact that
we needed to pass the monitor address at load time. This was a
bit bizarre anyhow, since it's really the Run RPC function that
needs this information. So, to enable caching and eager loading
-- which we need in order to invoke GetRequiredPlugins -- the
"phone home" monitor RPC address is passed at Run time.
In a subsequent change, we will switch to faulting in the plugins
that are missing -- rather than erroring -- in addition to
supporting the `pulumi plugin install` CLI command.
c04341edb2