Customisable Cycle Analytics - Single label selector

What does this MR do?

As part of #13076 (closed), adds a single label dropdown, the dropdown takes a list of labels and emits events when clicked and also when cleared.

Note: Selected label styling

I originally intended to use the label component from gitlab ui to match the designs when an option is selected, but with the introduction of the color property validator the component now expects to receive the color as a string not hexadecimal. This component will be used to render labels retrieved from the backend which currently return the color values as hexadecimal.

Screenshots

Default state	With a selection	With a long label	Dropdown open

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Closes #13076 (closed)