Store additional context as part of extras

What does this MR do and why?

Store and retrieve Duo Chat additional context as part of extras param.

That's not a breaking change, since all the additional context feature is WIP and guarded by a feature flag

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Open http://gdk.test:3000/-/graphql-explorer
2. Create a message

   mutation chatAction {
     aiAction(input: {chat: {content: "Explain this code", additionalContext: [
       {
         category: SNIPPET, id: "hello world", content: "puts 'Hello, world'",
         metadata: "{ \"freeform\": \"json\", \"e\": 1 }"
       }
     ]}}) {
       clientMutationId
       errors
       requestId
     }
   }

3. Read messages

   query getMessages {
     aiMessages {
       edges {
         node {
           id
           content
           extras {
             additionalContext {
               category
               id
               content
               metadata
             }
           } 
         }
       }
     }
   }

Related to #489304 (closed)