allow interface/[]interface types to be encoded if interface is struct type

[Alma-media]

We use HCL encoder to build dynamic configs for our services, unfortunately, there is no ability to use different types in the block. Let's say we have App config with additional options and couple of possible types:

type App struct {
	. . .
	Option []interface{} `hcl:"option,block"`
}

type HTTPOptions struct {
	Listener string `hcl:"listener,label"`
	Address  string `hcl:"address"`
	Secure   bool   `hcl:"secure"`
}

type MQTTOptions struct {
	Listener string   `hcl:"listener,label"`
	Address  string   `hcl:"address"`
	Topics   []string `hcl:"topics"`
}

We would like to be able to provide all of them:

var config = App{
	Options: []interface{}{
		HTTPOptions{
			Listener: "http",
			Address:  ":8080",
		},
		MQTTOptions{
			Listener: "mqtt",
			Address:  ":1883",
			Topics:   []string{"foo", "bar"},
		},
	},
} 

Desired output:

option "http" {
  address = ":8080"
  secure  = false
}
option "mqtt" {
  address = ":1883"
  topics  = ["foo", "bar"]
}

Or maybe someone could suggest more elegant way :thinking:

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