Draft: Add pagination to graphql branch rules type

What does this MR do and why?

This change adds pagination to the graphql BranchRuleType.

The implementation creates a new "finder" class that combines two types of branch rules (custom rules and protected branch rules) and serves them in paginated chunks.

References

Related to #576206

How to set up and validate locally

1. Create a number of branch rules in a project

2. Query the branch rules and use pagination

   {
     project(fullPath: "gitlab-org/gitlab-test") {
       name
       branchRules(first: 20) {
         nodes {
           name
         }
         pageInfo {
           hasNextPage
           endCursor
         }
       }
     }
   }

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