Werk #15657: Agent controller: Changes in the output format of the status command

This werk is only incompatible for users who process the json output of the agent controller status command (cmk-agent-ctl status --json). For all other users, this werk is purely informational.

The human-readable output of the status command has changed as follows:

• The section about the remote status of a connection does not report `Registration state: operational` anymore, since this was redundant information. If the host is registered, we now simply report hostname and connection mode.
• If the host is not registered anymore at the monitoring site, we now report `Not registered`. Before, the controller reported a 404 error with a description stating that the host is not registered.

These changes are also reflected in the new json schema for the remote status, which reads

{"status": "Registered", "hostname": "...", "connection_mode": "..."}

for registered hosts and

{"status":"NotRegistered"}

for hosts which are not registered.

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