Support clusters that can't see GitLab
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## Release notes
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## Problem to solve
As a self-managed GitLab user, I need to deploy to my in-cloud clusters that can not access GitLab directly. On the other hand, GitLab can access these clusters.
## Proposal
For self-managed GitLab users allow to enable a feature flag/config option that would make it possible to provide agent's URL when registering an agent.
kas can connect to such agents:
- to establish reverse tunnels so that agents can talk to kas (e.g. to pull configuration, etc).
- to make requests for CI tunnel functionality.
Technical design is in https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/master/doc/kas_to_agentk_connectivity.md.
## Intended users
- Infrastructure Engineeer
- Self-managed only
## Tiering
~"GitLab Ultimate" only as this serves users with high security requirements who are restricted from creating a VPN with their cloud provider. We expect such company-wide regulations to come from executive levels. See the [GitLab buyer-based pricing model](https://about.gitlab.com/company/pricing/#buyer-based-tiering-clarification) for more details.
## Metrics
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### References
- https://gitlab.com/groups/gitlab-org/configure/-/epics/8+
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