refactor: organize plugin to support multiple services (#1)

Co-authored-by: Michael Li <michael.li@hashicorp.com>

plugin/service/host/const.go

@@ -0,0 +1,10 @@
+package host
+
+const (
+	// ConstDescribeInstancesFilters is the key for the filter in the dynamic host set.
+	ConstDescribeInstancesFilters = "filters"
+
+	// ConstInstanceNameTagKey denotes the special AWS tag key that contains an EC2
+	// instance's name. This field is to be matched literally.
+	ConstInstanceNameTagKey = "Name"
+)