Add WorkspacesAgentConfigSettings step to the Workspaces Settings Module
<!--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=479700)
</details>
<!--IssueSummary end-->
MR: Pending
**NOTE: We need to decide on the proper field, `max_hours_before_termination_limit` may not be the best choice anymore**
## Description
Add `WorkspacesAgentConfigSettings` step to the Workspaces Settings Module. The related UI for editing agent config settings will be added by https://gitlab.com/gitlab-org/gitlab/-/issues/457760+
- See https://gitlab.com/groups/gitlab-org/-/epics/14186+ for overview and more context
- See [Related Issues and MRs section in that epic](https://gitlab.com/groups/gitlab-org/-/epics/14186#related-issues-and-mrs) for related issues and MRs
## Acceptance Criteria
* [ ] Decide the appropriate field. See https://gitlab.com/groups/gitlab-org/-/work_items/14186#note_2066748871 for relevant discussion.
* [ ] Add step for `WorkspacesAgentConfigReader`, after `CurrentSettingsReader`
* [ ] Fully exercise new behavior in `spec/lib/remote_development/settings/settings_integration_spec.rb`
* [ ] Add any places which directly read from table to instead read from settings module.
* [ ] Adds documentation
## Technical Requirements
- This will extend support for `max_hours_before_termination_limit` to the agent config level.
- This will involve adding a new AgentSettings step to the Workspaces ROP Settings Module chain
- This doesn't involve creating any GraphQL API for that field, that will be done as part of the following UX issue
## Design Requirements
None, no UI-facing changes.
issue