Support for project level config key

[AaronFriel]

Pulumi has gained support for a project-wide config key (pulumi/pulumi#10832).

As YAML is a language host using the Go SDK, it should rely on on the Go SDK config package (GetConfig and IsConfigSecret) to derive configuration keys, and use the structure of the values to infer their types, leaving validation up to the engine. If that's not possible, we should should use shared code from the engine to parse the type and items fields.

As a consequence of using GetConfig, it may not be possible to statically analyze a Pulumi program absent a stack file to determine if the a key is defined. We may want an enhancement to the Go SDK's config package to allow us to query for the existence of a key or for static analysis to rely on the engine's parsing of the project file to be consistent.

As this would be a breaking change for configuration declarations; we should support the old key with a warning to avoid breaking users prior to GA.

[AaronFriel]

Duplicate of #369, created from my to-do list as a lapse.