- Add component ref coverage to the existing test
- Add coverage for a downlevel SDK communicating with an engine that
supports resource refs
- Add coverage for a downlevel engine communicating with an SDK that
supports resource refs
As part of improving coverage, these changes add a knob to explicitly
disable resource refs in the engine without the use of the environment
variable. The environment variable is now only read by the CLI, and has
been restored to its prior polarity (i.e. `PULUMI_ENABLE_RESOURCE_REFERENCES`).
