Add `valid_roles` property to group members

What does this MR do?

Part of &4233 (closed) and #235603 (closed)

I am working on converting the group members page from HAML to Vue. As part of this initiative the frontend needs to know what roles are available to each member. This MR updates the JSON passed to Vue to include a valid_roles property. These roles will be used in the dropdown that allows the admin to change a member's role.

Currently this is behind the vue_group_members_list feature flag. To enable:

1. bin/rails console
2. Feature.enable(:vue_group_members_list)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind a feature flag
• [-] 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