VSA filter bar additional tokens

What does this MR do?

This MR is part of #217943 (closed), it adds:

• UserToken, a shared token to be used for both the assignees and author filters
• Adds all the additional author and assignee tokens to the VSA filter bar
• Updated specs

NOTE: fetching data to populate the tokens and persisting them in the URL will be in a follow up MR. You will also need to enable the value_stream_analytics_filter_bar to view the filter bar.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Related to #217943 (closed)