Fix filter bar member request for subgroups

What does this MR do?

• Adds a currentGroupParentPath getter to the VSA vuex getters
• For subgroups, we want to fetch the group members of the parent group

NOTE: Currently behind the value_stream_analytics_filter_bar feature flag, so no changelog entry added.

From the discussion

So I'm testing on gitlab-org/manage VSA and had a question: would we expect very few autosuggest options? For example, with author the only autosuggest options were the 2 owners of the project:

vsa-filters-not-many-autosuggest-options

Testing instructions

The filter bar requires the value_stream_analytics_filter_bar feature flag and only applies to group level VSA (currently not project level)

1. Navigate to any sub-group
2. From the left side bar Navigate to Analytics > Value stream analytics
3. Select a project from the dropdown, then the filter bar can be tested
4. Query for a author or assignee using the filter bar
5. Repeat on master and you will see less results returned

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