Add contract test for suggested reviewers dropdown

What does this MR do and why?

Adds a contract test for the Suggested Reviewers feature, specifically the dropdown element that appears in a merge request that lists suggested reviewers, and the backend endpoint that provides the data..

• Adds new dev packages to packages.json and associated changes to yarn.lock.
• Adds a script jest:contract to packages.json to run the consumer contract tests.
• Adds a rake task contracts:merge_requests:pact:verify:suggested_reviewers to run the provider contact tests.

Note: Contract tests don't run in CI yet. Work to enable that will be tracked in #370405 (closed)

Part of gitlab-org/quality/quality-engineering/team-tasks#1410 (closed)

How to set up and validate locally

To run the consumer tests:

yarn install
yarn jest:contract ee/spec/contracts/consumer/specs/project/merge_request/show.spec.js

To run the provider tests:

bundle exec rake contracts:merge_requests:pact:verify:suggested_reviewers 

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.