Add a CharmInfo API command.

state/api/params/params.go

 package params
 
 // ServiceExpose holds the parameters for making the ServiceExpose call.
 type ServiceExpose struct {
         ServiceName string
 }
 
 // ServiceSet holds the parameters for a ServiceSet
 // command. Options contains the configuration data.
 type ServiceSet struct {
@@ skipping 52 matching lines @@
         DeployerName string
         // TODO(rog) other unit attributes.
 }
 
 // User holds details of a user.
 type User struct {
         // This is a placeholder for any information
         // that may be associated with a user in the
         // future.
 }
+
+// CharmInfo stores parameters for a CharmInfo call.
+type CharmInfo struct {
+        CharmURL string
+}