These changes add support for unmarshaling and marshaling package
schemas using YAML instead of JSON. Language-specific data is
canonically JSON. Users of the `*Spec` types will need to update the
types of the the their `Language` values to use the new
`schema.RawMessage` type instead of `json.RawMessage`: the former has
support for YAML while the latter does not.
Rather than duplicating the list of tests and codegen driver across each
SDK, move its definition into `pkg/codegen/internal/test`. This has a
few notable benefits:
- All SDK code generators will be tested against each test. Though some
tests may exercise a particular code generator more than others, the
extra coverage will be generally beneficial.
- Adding a new test is simpler, as only a single file needs to be
changed.
- All SDKs now honor the `PULUMI_ACCEPT` environment variable for
updating baselines.
- Codegen tests now validate all generated files instead of only a
particular subset.
