Add support for plugin protocol v6

[mildwonkey]

This PR adds a new package, tfprotov6. It is mostly copy-pasted from tfprotov5. The interesting changes (besides just swapping 5 for 6 in imports everywhere) is the new tfprotov6/internal/tfplugin6/tfplugin6.proto file, taken directly from terraform's main branch, an assortment of renamed functions (ValidateResourceTypeConfig is now ValidateResourceConfig, etc - all the changes we discussed internally), and finally the interesting one, the new NestedType field inside an Attribute (with associated conversions in toproto and fromproto).

If you want to see any of this in action, I have a very tiny implementation of the random provider with this branch vendored: https://github.com/mildwonkey/terraform-provider-random

I also have a fork of ptyng's sql provider using this; it works as a provider just fine but I'm still struggling with the acceptance tests, which required changes to the sdkv2. I'll keep poking at that; it'd be nice to show off a provider using these new features with working tests.

[paddycarver]

I think, as future work, we're going to want to refactor the JSON/MsgPack encoding out of the protocol-versioned packages and into tftypes, so we don't need to keep implementing them for each new protocol version, but I don't think it's reasonable to lump that in with this PR.

LGTM. 👍 Thanks!

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