* Add support for plugin protocol v6
This PR turns on support for plugin protocol v6. A provider can
advertise itself as supporting protocol version 6 and terraform will
use the correct client.
Todo:
The "unmanaged" providers functionality does not support protocol
version, so at the moment terraform will continue to assume that
"unmanaged" providers are on protocol v5. This will require some
upstream work on go-plugin (I believe).
I would like to convert the builtin providers to use protocol v6 in a
future PR; however it is not necessary until we remove protocol v6.
* add e2e test for using both plugin protocol versions
- copied grpcwrap and made a version that returns protocol v6 provider
- copied the test provider, provider-simple, and made a version that's
using protocol v6 with the above fun
- added an e2etest
