Rename common gitlab-agent endpoints in Rails
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MR: Pending
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## Description
The following discussion from !194418 should be addressed:
- [ ] @vtak started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/194418#note_2560376517): (+1 comment)
> **question; non-blocking**
>
> Should we consider introducing a new endpoint which does not have the `kubernetes` part in a followup? This endpoint is being called from KAS only AFAICT.
>
> /cc @cwoolley-gitlab @ash2k
## Acceptance criteria
TODO: Fill out (required)
- [ ] [Describe what must be achieved to complete this issue.]
- [ ] [If applicable, please provide design specifications for this feature/enhancement.]
- [ ] [If applicable, please list any technical requirements (performance, security, database, etc.)]
## Implementation plan
Rename endpoints according to [comment below](https://gitlab.com/gitlab-org/gitlab/-/issues/549502#note_2563813164):
- internal/agents/... will be the entry point for all endpoints to be consumed by KAS and common modules across agents.
- internal/agents/agentk/... will be the entry point for all endpoints to be consumed by agentk only.
- internal/agents/agentw/... will be the entry point for all endpoints to be consumed by agentw only.
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