Add option to let user account start and expire at a given date/datetime
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=17632)
</details>
<!--IssueSummary end-->
### Description
Given you work in projects for short periods in time, it would be nice to have the ability to define an end of usage per user. This may also make it easier for small companies to understand their flow of users through GitLab. For example, we have 40 users inside, where 5 are constantly working, and some of the 40 have never or very rarely GitLab.
### Proposal
* Add 2 new fields start and enddate into user account
issue