Feat(key_contacts_management): Add feature flag

What does this MR do and why?

This MR adds the key_contacts_management feature flag to GitLab.com using this process, so we can update the purchase flow for customers with a billing account: &12058 (closed)

1. Feature flag: [Key Contacts Management] Create the key_contac... (#432400 - closed)
2. Rollout issue: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5873+
3. CustomersDot feature flag: https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags/195/edit

Work that will depend on this feature flag is being implemented, so would appreciate a speedy review process 🙏 : Follow-up: Contact Management (&12058 - closed).

Changelog: added EE: false

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

No visual change, yet. Just adding the feature flag in.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:key_contacts_management)

2. Visit any page in GitLab.com, e.g. http://localhost:3000, or http://localhost:3000/-/subscriptions/new?plan_id=2c92c0f876e0f4cc0176e176a08f1b70&test=capabilities
3. Open the console and run to see it enabled for your user

   window.gon?.features.keyContactsManagement

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.

Related to #432400 (closed)