Add lastActivityOn field to GraphQL Member type

What does this MR do and why?

In order to provide the data necessary for the page that lists the data for a custom role we need additional fields to be exposed via the GraphQL API.

#489771

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Before	After

How to set up and validate locally

1. Navigate to GraphQL Explorer http://gdk.test:3000/-/graphql-explorer
2. Enter the following query:

   query findUsers($active:Boolean!) {
     users(active: $active) {
       nodes {
         groupMemberships{        
           nodes{
             accessLevel {
               stringValue
             }            
             lastActivityOn
           }
         }
       }
     }
   }

3. Enter the following variables:

   {
     "active": true
   }

4. Execute the query