feat(datepicker): add public show method for datepicker

What does this MR do?

!3062 (merged) revealed that there are consumers of ~"component:date-picker" which called .calendar.show() to programmatically trigger dropdown

To avoid relying on component internals, we're adding show() as method directly on datepicker

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: gitlab!101435 (closed)
  • CustomersDot: N/A
  • Status Page: N/A
• Added the ~"component:*" label(s) if applicable.