plugin2host: Send marked values over the wire

[wata727]

See also #235

For similar reasons as #235, values marked sensitive were not sent to plugins. In particular, there is a technical difficulty that marked values cannot simply be serialized.
https://github.com/terraform-linters/tflint/blob/v0.45.0/plugin/server.go#L128-L137
https://github.com/zclconf/go-cty/blob/v1.13.0/cty/msgpack/marshal.go#L46-L48
https://github.com/zclconf/go-cty/blob/v1.13.0/docs/marks.md#marks-under-serialization

This PR serializes sensitive values in a proprietary way so that they can be sent to plugins. Below are compatibility notes:

• Before TFLint v0.46
  • Both server-side and client-side are checked for errors. However, the added error is never returned as the error is always returned on the server side only.
• TFLint v0.46
  • An error is returned on the client side only. However, for cty.DynamicPseudoType it now returns the marked value instead of an error.

Be aware that the TFLint v0.46 + SDK v0.15 can unintentionally expose sensitive values to plugins. To avoid this, the server should return sensitive values only for SDK 0.16+.