Some properties have constant values, such as the
apiVersion and kind for the Kubernetes provider. Add
a Const field to the Property schema to support this.
Replace the various `defaults` maps in the schema with per-property
`default` and `defaultInfo` fields. The former holds the static default
value; the latter holds the envvars and language-specific info.
Also, fix a minor bug in the Python codegen that caused diffs in
property docstrings.
The schema format is described in pkg/codegen/schema/schema.go. The code
generators are derived from the code generators contained in
https://github.com/pulumi/pulumi-terraform-bridge/pkg/tfgen, with the
exception of the Go code generator, which is net new.
