Add approvalRules to MergeRequest GraphQL API

What does this MR do?

This change adds new field to MergeRequest GraphQL API that allow users to fetch information about MR approvals.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Create new project.
2. Go to Settings -> Merge request (MR) approvals
3. Configure at least one approval rule.
4. Create at least one MR for created project.
5. Go to GraphQL Explorer.
6. Use query to fetch information about the rules for given Merge Request:

query {
  project(fullPath: "root/new-project") {
    id  
    mergeRequests {
        nodes {
          iid
          approvalState {
            approvalRulesOverwritten
            rules {
              approvalsRequired
              approved
              section
              approvedBy {
                nodes {
                  id
                  name
                }
              }
              containsHiddenGroups
              eligibleApprovers {
                nodes {
                  id
                }
              }
              groups {
                nodes {
                  id
                  name
                }
              }
            
              name
              id
              overridden
              sourceRule {
                id
                name
              }
              type
              users {
                nodes {
                  id
                  name
                }
              }
            }
          }
        }
      }
    }
  }

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #323116 (closed)