feat(GlDropdown): Allow custom button text content while still keeping default loading spinner, icon, and dropdown caret

What does this MR do?

<b-dropdown> in Vue Bootstrap has a button-content template that allows custom content for the button's text while still showing the default dropdown caret:

With <gl-dropdown>, we've added some additional features: a loading spinner, an icon, and a different dropdown caret. However, the same button-content template is an all-or-nothing deal; you can either keep the default loading spinner, icon, and caret with text passed in through the text attribute, or the button-content template will replace everything in the button. This means that if you only want to customize the button content but keep the default icons, it's impossible to do so without adding them back in manually.

This MR adds the ability to pass in custom content for only the text portion of the button, while still keeping the default loading spinner, icon, and caret:

<gl-dropdown>
  <template #button-text>
    Custom <strong>Content</strong> with <em>HTML</em> via Slot
  </template>
</gl-dropdown>

Note that this still allows the button-content template to be used, and the behavior for it has not changed.

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!57832 (closed)
  • 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.