feat: Create GlTextInputDropdown

What does this MR do?

This MR adds a GlTextInputDropdown component which could eventually replace a lot of complexity in the header_search/components/app.vue and the complexity being added in gitlab!97343 (merged).

Instead of recreating complex dropdown positioning logic, we can actually reuse Bootstrap's dropdown mixin. For simplicity, we opt to create an internal component to encapsulate this usage of Bootstrap's mixin.

Screenshots

Desc	Screenshot
Demo	20220924_text_input_dropdown_demo
🎉 Same styling as a regular ol' dropdown, but now it's triggered by an input!
🎉 Consumers of GlTextInputDropdown can change dropdown based on input change
🎉 Using keyboard to select from dropdown works nicely and scrolls selected item in view
🎉 Dropdown will automatically reposition if it will overflow off screen

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
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.