Add attribute flag building and parsing (#1562)

This adds a few command sketches but that's not the main point of the PR
and filling those out will come in a different PR. This adds a new flag
type that can combine inputs to a single target slice, retaining the
ability to distinguish where it came from but also keep ordering across
different flag names. It also implements a function that does the
heuristic parsing with fallback.

internal/proto/local/controller/api/resources/hostsets/v1/host_set.proto

@@ -52,7 +52,7 @@ message HostSet {
 	repeated string preferred_endpoints = 101 [json_name="preferred_endpoints", (custom_options.v1.generate_sdk_option) = true];
 
 	// The attributes that are applicable for the specific Host Set type.
-	google.protobuf.Struct attributes = 110;
+	google.protobuf.Struct attributes = 110 [(custom_options.v1.generate_sdk_option) = true];
 
 	// Output only. The available actions on this resource for this user.
 	repeated string authorized_actions = 300 [json_name="authorized_actions"];