[Time tracking] Update issue spend and estimate times
### Problem to solve
This is an extension of the closed issue #934.
Looks like it's possible to setup the estimate and time spent on the creation of a ticket.
But those options are not present on the `glab issue update` counterpart.
Only way is through the web-page or API calls.
### Proposal
Same features as using slash-commands on the discussion text-box on an issue page.
| Slash | CLI option | Behaviour |
| ------ | ------ | ------ |
| /spend | --spend | Add given time to current spent time |
| /remove_time_spent | --remove-time-spent | Reset spent time |
| /estimate | --estimate | Set given time as estimate |
| /remove_estimate | --remove-estimate | Reset estimate |
### Further details
- This would be useful when working on an issue and report the time spent through the CLI.
- Allows scripting with `glab`
### Links / references
[Update command options](https://gitlab.com/gitlab-org/cli/-/blob/main/docs/source/issue/update.md)
issue