Support for Protobuf `Any` type in gRPC DSL

[ilCollez]

When defining a Goa service with gRPC, using the Any type in the payload results in an error, even though Protobuf provides native support for Any via the Well Known Types library

Example Code:

package v2

import (
	. "goa.design/goa/v3/dsl"
)

var _ = Service("example", func() {
	Description("An example service")

	Method("hello", func() {
		Description("Says hello")

		Payload(func() {
			Field(1, "object", Any, "Object to greet")
		})

		HTTP(func() {
			GET("/hello")
		})

		GRPC(func() {
			Response(CodeOK)
		})

		Result(String)
	})
})

Error:

service "example" gRPC endpoint "hello": Attribute "object" is Any type which is not supported in gRPC

Expected Behavior:

Since Protobuf already supports the Any type, Goa should ideally allow the use of Any in gRPC payloads and map it to google.protobuf.Any.

Question/Feature Request:

Can support for Protobuf Any be added to Goa’s gRPC DSL so that attributes declared as Any map to google.protobuf.Any in the generated .proto files?

[raphael]

Makes sense, I'd be happy to merge a PR that added this capability.