Turn on helper/schema proto5 flag in GetSchema
Turn on helper/schema proto5 flag in GetSchema

This turns it on at the last moment, and in one place for all uses of
helper/schema. There's no way to use the new protocol without calling
GetSchema, so we can be sure that any subsequent api calls have this set
when required.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk