* providers.Interface: huge renamification
This commit renames a handful of functions in the providers.Interface to
match changes made in protocol v6. The following commit implements this
change across the rest of the codebase; I put this in a separate commit
for ease of reviewing and will squash these together when merging.
One noteworthy detail: protocol v6 removes the config from the
ValidateProviderConfigResponse, since it's never been used. I chose to
leave that in place in the interface until we deprecate support for
protocol v5 entirely.
Note that none of these changes impact current providers using protocol
v5; the protocol is unchanged. Only the translation layer between the
proto and terraform have changed.
plugin6 includes a `convert` package to handle conversion between the
plugin protocol and configschema, and the GRPCProviderPlugin interface
implementation for protocol v6.
