Add expires_before arg and count support to user personal access tokens GraphQL endpoint

What does this MR do and why?

1. Add expires_before argument to user { personalAccessTokens } GraphQL field to enable filtering for tokens that expire before a specified date
2. Support user { personalAccessTokens { count } } GraphQL query

References

[BE] Get PAT statistics from GraphQL (#581649).

Screenshots or screen recordings

How to set up and validate locally

1. Login with any user
2. Create a personal access token for the user
3. Go to http://localhost:3000/-/graphql-explorer and run the following query

   query {
     user(id: "gid://gitlab/User/<user_id>") {
       personalAccessTokens(state: ACTIVE, expiresBefore: "2099-12-31") {
         count
       }
     }
   }

4. Verify that the query succeeds and count field is present

MR acceptance checklist

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