Allow GlDisclosureDropdown to always open down

Proposal

In some instances, we want a dropdown to always open below the trigger button unless there's no room to open it, rather than allowing autoPlacement to determine if there's more room above/below the button. Example issue: gitlab-org/customers-gitlab-com#7046

Possible solutions

• Allow crossAxis to be set to false
• Allow for additional placement/alignment options (ex: right-bottom)