Add external_url field to Environment GraphQL Type

What does this MR do and why?

This adds the externalUrl field to the Environment GraphQL Type. Adding this prevents the need for using GraphQL and Rest to get the externalUrl based on an environment name

Screenshots or screen recordings

How to set up and validate locally

1. Checkout the branch

2. Log in to GDK

3. Make sure the chosen environment has an external url

4. Execute this query in GraphiQL

   query envUrl {
     project(fullPath: "flightjs/flight") {
       environment(name: "production") {
         name
         path
         externalUrl
       }
     }
   }

MR acceptance checklist

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