Fix managed gateway stack error

[vlasebian]

Summary

References issue #4423.

Changes

• Add disabled managed gateway config server
• Use disabled managed gateway config server when ttgc is not enabled

Testing

Steps

• Run the stack locally with ttgc disabled
• Add a dummy gateway or make sure one is available
• Send the following request (change ttig with the id of the dummy gateway)

  curl -s --location  -H "Authorization: $TTN_AUTH_TOKEN" "http://localhost:1885/api/v3/gcs/gateways/managed/ttig\?field_mask\=wifi_profile_id,ethernet_profile_id,version_ids,wifi_mac_address,ethernet_mac_address"

Results

The request should return an error that says the managed gateway config server is disabled.

Regressions

None.

Notes for Reviewers

None.

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• [ ] Documentation: Relevant documentation is added or updated.
• [x] Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• [ ] Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• [ ] Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• [x] Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[vlasebian]

Tested this in the console too, the page is loaded successfully, there is only a 404 Not Found in the network tab as expected.