This PR is best reviewed commit by commit:
- bbe1fc6b2f exchanges `ParseTolerant` for
`Parse` in `Parameterize`.
- 76df18be56 updates the `Parameterize`
interface for forward compatibility.
- 746c057668 requires implementors of
`plugin.Provider` to make a forward compatibility choice explicitly.
This is similar to what we require for gRPC already. Since this will
release with bbe1fc6b2f, consumers will
already need to update their `plugin.Provider` implementor, minimizing
the disturbance.