Frontend: Add group secrets manager to navigation menu

Why are we doing this work

When the group secrets manager is enabled in a group, the user should be able to find it in the navigation menu under Secure > Secrets manager

Note that there's a process for Requesting a Navigation Change, so we should get approval for this first.

This depends on #577451 (closed) being completed first so that the routes/page is set up.

Relevant Links

Add Secrets Manager sub-menu option under Secure for project group secrets.

Implementation plan

Verification steps

1. Upload a Premium license (or above).
2. Set up openbao and enable the secrets manager on your gdk.
3. Enable the group_secrets_manager feature flag.
4. Go to your group settings (Settings > General) and expand Permissions and group features.
5. Toggle the group secrets manager and refresh the page.
6. Verify that going to Secure > Secrets manager from the navigation menu leads to /path-to-group/-/secrets.