[SaaS UserCap FE]: Add "Approve" button to pending members table
## Description
In #333064 we listed the pending members awaiting approval in the Seat usage > Pending members tab.
Now we want to add an "Approve" button to each pending member row.
### Pending members table
| Issue | Concerns |
| ------ | ------ |
| #333064 | It only adds the table and list the users with badges |
| #330027 (:point_left: you're here)| It adds the "Approve" button to each row in the table |
## Feature flag
This feature will be implemented under the `saas_user_caps` FF.
## Designs
:warning: _New Design_ :point_down_tone3:
https://gitlab.com/gitlab-org/gitlab/-/issues/321934/designs/pending-alert.png#note_735382597
issue