Enhancements for `issuable_list` app

What does this MR do?

This MR updates issuable_list app with following enhancement;

• Custom reference and status support via slots.
• Assignees support.
• Labels permalink support.
• Ability to update page URL using provided urlParams prop.
• Tab counts are now optional.
• Handle external URLs for Issuable items.

Screenshots (strongly suggested)

Changes done in this MR do not visually affect any existing usage of issuable_list app, as this enhancements are primarily aimed at !45678 (merged).

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team