Allow to pass an EE license when installing the chart

Summary

GitLab has a production seed file to populate the EE license from a GITLAB_LICENSE_FILE env variable or from a Gitlab.gitlab-license file: https://gitlab.com/gitlab-org/gitlab/blob/4055c2fe16792220e74b28b45922ee13f8150a2a/ee/db/fixtures/production/010_license.rb#L3-4

The chart should be able to take a new gitlab.migrations.license command line option similarly to the gitlab.migrations.initialRootPassword.secret option.

Passing this option would:

1. Pre-create the secret containing the license file
2. If set, mount to the appropriate path within the migrations Job's Pod.

Note: As we'd be making use of the file-based pattern rather than the environment, no change would be needed to the scripts within gitlab-rails.