Add a hosted/managed Unleash proxy service for GitLab SaaS Customers to use Feature Flags at Scale

Proposal

I met with an Ultimate SaaS customer that wants to see a hosted/managed Unleash Proxy service from GitLab so they can take full advantage of the Feature Flags functionality. They do not want to manage their own instance and feel like this should be included as part of their paid subscription.

Unleash provides a hosted proxy as an option to their paid customers on a managed Unleash instance: https://docs.getunleash.io/topics/proxy-hosting#unleash-hosted-or-self-hosted

This is only available to Pro and Enterprise customers who have signed up for a managed Unleash instance.