feat(GlDropdown): Add show method to GlDropdown

What does this MR do?

This adds a show method to the GlDropdown component

Rationale

Due to a requested behavior from gitlab!28603 (comment 330909890), we need to have a way to call the show event from the wrapped BDropdown component, adding the show method helps us accomplish this, so we can show the dropdown from other contexts. The above merge requests calls this when a control listen for keypress.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.